It wasn't that simple IIRC. The Tyrells are noted to be the Great House with the weakest hold over their bannermen due to not being a royal house pre-Conquest (in fact many of their vassals, like House Florent to whom Stannis has marriage ties, have better claims to the Reach on account of their superior ties to the extinct royal House Gardener), even in the present day Olenna Redwyne had to worry about them being challenged for the paramountcy by their vassals,
and there was a real danger of the pro-Stannis Reachmen led by his Florent in-laws flipping the allegiance of many or even most of the Reach troops, which took some decisive action by Loras & Randyll Tarly to suppress. Yes, of course we now know the Tyrell loyalists won that round in the end, but in the immediate aftermath of Renly's death Stannis did have some reason to think he could get the Reach (or at least, a big enough part of it to cushion any curbstomping planned by his enemies; half, or even just a quarter, of those 80k Reachmen would have gone a long way to evening the odds he faced) onto his side, and thus actually have a shot at the IT.
If that is the case, then Stannis should have focused on getting the Reachmen to his side first, and flushing out anyone who might disagree, like Loras and Randyll. Then once his hold over the Reach is secure, Stannis can then send the massive Reach army to King's Landing. No amount of Tywin coming to the rescue is going to stop THAT.
If anything, Stannis was foolish to let Loras and Randyll live, since those two could easily lead a counter-attack even if his attack on King's Landing was a success. Sure, he'd get to Joffrey and Cersei and kill them, but with Loras and Randyll nipping at his heels, he'd still get fucked, especially once Tywin showed up.
The Redwynes also genuinely had good reason for not immediately springing into action, their lord's twin sons were being held hostage at KL when hostilities first erupt so they didn't mobilize or really do anything whatsoever to support their overlord's pledging to Renly at the beginning of the Wo5K. Then after the Tyrells and Lannisters reconciled, it took said Lord Paxter too much time to pull together & then move their 200-ship navy towards the Narrow Sea; he does actually participate in blockading Stannis' homebase, but by then Stannis has already long since fucked off to the Wall.
That made no fucking sense. After the Tyrells and Lannisters reconciled, they should've immediately moved to combine their naval assets to take out Stannis first. Especially since it was Stannis who threatened the capital, not Robb Stark who just wanted to be left alone with the North as a free entity.
Now there's no argument to be had that the shadowbabies could have been used more intelligently. At the very least, the second one could've been used against Tywin or Randyll Tarly instead of being wasted on the castellan of Storm's End (who could have been taken down in a duel, which he himself proposed, with any of Stannis's champions). I think in hindsight it's just a case of Martin needing his characters to squander their obvious overwhelming advantage for plot reasons, like Arya wasting Jaqen's abilities on some random insignificant niggers at Harrenhal instead of someone like Tywin but way more retarded & out-of-character (since Arya at least has the excuse of being a traumatized 9-year-old girl who's way in over her head, while Stannis is a grown adult & experienced strategist).
Martin obviously wasn't planning ahead. Him describing himself as a gardener instead of an architect, forgetting the fact that gardeners are the ultimate architects, since they work with organic materials, so they have to plan out how plants grow and how far to trim the hedges or weeds.
If I was Stannis, like I said, either I'd embrace Renly as my king and get into his good graces, then have the shadow baby kill him and blame the Lannisters for it, that way, I'd get even the Tyrells in my pocket, or I'd scare the shit out of Renly and the Tyrells by having the shadow baby pose as my "assassin" and kill Tywin. If someone that powerful, in a secure castle like Harrenhal, can be killed at my word, then Renly would realize that the only reason he's not dead is because I haven't killed him yet. Loras would want Renly to live since they're both gay for each other, and so, Renly would bend the knee, and I'd make him my heir to placate his Tyrell friends.
That's the thing though, Artois IRL did end up becoming an experienced captain (albeit still one less successful in the field than Stannis was in the books, his biggest battle being an early defeat for the English in the HYW). Sometimes niggas are just way too fucking stubborn for their own good & that of everyone around them; in that regard Stannis (especially pre-Blackwater Stannis, who's a much bigger asshole than post-Blackwater Stannis) is not particularly unrealistic.
He kind of is, because Stannis has been to war before, and he knows what it's like to starve inside a castle, surrounded by a large army feasting at his walls. The only thing stopping that from happening again was the fact that Tywin and the Tyrells had some Northern bugs to squash. So even pre-Blackwater Stannis had eaten a lot of humble pie back during the war against the Mad King.
The Lancastrians also lost in the end, but at the time of the Readeption they had some ground to think they could've succeeded, since they did get Warwick the Kingmaker (who was like a one-man English House Tyrell at the time, in that he was literally 15th century England's #1 landowner and managed to raise a total of 60,000 men over two years - 30k crushed at Losecoat Field in 1470, another 30k for Barnet in 1471) on their side. Unfortunately for them their opponent, Edward IV of the House of York, was the real-life basis for both Bobby B and Robb Stark (young, charismatic gigachad & military genius who never lost a battle, also almost lost his throne due to breaking a betrothal arranged by the Kingmaker but obviously ended up luckier than Robb in that regard), and so he kicked all their collective asses. But on paper, like Stannis right after whacking Renly but before the Tarlys & Loras destroyed the Florents' plot to subvert the Reachmen, they had good reason at the time to think their plan wasn't actually suicidal.
Then again, Stannis should've pressed on and defeated Loras and Randyll. Push the advantage and remove the Tyrells and Tarlys from the equation, or force them to surrender and submit. Make sure the Florents' plan succeeded, then Stannis can take a massive Reach-Stormlands army out to King's Landing, throw caution to the wind, batter down its walls in a land battle, or attack from the land and sea, with the Reachmen under the Florents attacking the city gates while the Baratheons at sea try to seize the mudgate on the shore. Tyrion can stop one, but not both, and they'd still have enough men to push Tywin back when he inevitably comes to reinforce Joffrey.
