A Lv50 Machamp holding a Choice Scarf with an optimal Nature and full IVs/EVs would have a Speed stat of 175. If we disregard the opponent Pokemon boosting their Speed via items, abilities, or moves, this is faster than the likes of Iron Treads, Urshifu, Chi-Yu, or Garchomp. With a neutral nature, the list would expand to include the likes or Ogerpon, Iron Moth, Iron Valiant, Roaring Moon, Sneasler, and so forth. So a decent chunk of Pokemon would get kneecapped because they either can't outrun Max Speed Machamp, or can't beat him in a single blow (compounded by the fact that Machamp can distribute its remaining 258 EVs into defensive stats).
Another issue is because of Scarf Fissure Machamp even existing, this means players must not only include at least one Pokemon in their team to take him out, they would also need to have that Pokemon ready when the big muscle man rushes out to sweep your party.
Yes, I said party, which leads to the third issue being the opponent Machamp would also be in a party, most definitely flanked by two Pokemon to deal with any threats so Machamp can drop Pokemon in chasms with no interruptions.
The whole situation would be like the Gen V metagame where any Pokemon slower than Breloom had no place to even start.
