I think you might have missed this bit:
After first doing an achievement that proves they’re good enough as a player to be “worthy” of 220 caps (not even 226 in its current design), they get the option on their alt to skip one achievement threshold and push 2 ranks past the 213 cap to 220 - if they’re getting a full set of 203 gear from M+10, they’re probably naturally going to get Keystone Conqueror depending on the spread of viable loot drops, so that gap is the only actual difference you’re talking about.
Basically your entire premise is not talking about alts that are 200 ilvl vs alts that are ~220, it’s comparing alts that are naturally going to have a mix of 213/220 to alts that might have full 220.
The more you weigh that to the 220’s from the GV as opposed to loot drops, the less anything is “altered”, so the less anything is being “affected”.
I have not raided much this expansion at all. How far will a 200 ilvl get in Mythic CN?
There really isn’t a limit for alts. Without bringing alts in for farm, bunches of gear will go to waste, the alts will help stack loot drops into the right hands in future tiers (especially if they have the same ilvl as mains to make the “trade” limitation go away), and they keep people from burning out.
Back in the day, I had alts that were full BiS, or were killing everything except the super end-boss of the tier, and we even occasionally had the odd alt accidentally be in for the first progression kill when we were using them to plug attendance gaps and almost ended up killing a boss we weren’t expecting to kill that night.