I actually disagree on this one. It would make completing the final milestones way to fast (too little effort) for those with a bunch of 6*). And if you say “well just add more milestones”, like they did in T4Basic, you know lower people will still want to go after them all (and you’ll basically be encouraging them to spend a whole lot more time grinding for these that they should have to do, just because they don't have a lot of 6*).But also (mainly) keeping 6* out of these arenas allow lower level players to better be able to compete against Thanos/Kang Teams of Max 5* (NO Mastery) opponent’s (instead of max 6* T/K opponent teams), if they are not aware of using “unbalanced” teams to reach Infinite Streak at 15. Also helps to keep the minimum PI requirement a little lower once in Streak (as opposed to if higher 6* PI's were to be introduced into that calculation.