Hirelings are not an alternative to real players, I think that's your big mistake. There is no reason that a soloist is entitled to full party play by themselves, and I say that as a soloist.
I consider it axiomatic that it's better for the game if players play it, so it's better to have six players playing it than one player plus the game playing itself
Plus, again, we soloists are already able to fully access the game solo, outside of raids. So you're not asking for accessibility, you're just asking for the increased rewards of playing higher difficulties that parties can achieve, while the game plays itself for you.
And again, balance is difficult to achieve. Giving up a little power of your own to grant that much power to five hirelings is not an even trade, and potentially makes Summoners way OP if you go all the way