All allies is, is you get a small amount of bonus xp for being in a party with others, all it really does is it encourages people to play with friends more instead of playing with randoms all the time.
Nope you will still have people who prefer to play solo, and to be honest the xp boost isnt huge so its nothing to worry about.
You dont loose any progress made on the allies system so if you and your friends only play together once a week you will still gain more progress to your next allies rank.
They know playing the game is more fun with friends so maybe it’s just their way of preserving the playerbase. Friends peer pressuring each other into playing together because of this xp boost thing. I don’t see why they would personally care if we played with randoms or with friends. Other than to help keep the playerbase up.
I really doubt it will increase the prevalence of stacked teams in ranked. Maybe it will with lower ranks/more casual players just trying to have fun. But hopefully the matchmaking is such that it won’t matter when solo queueing.