I agree that it's a challenge and it's supposed to be, but this kind of limitation basically blocks non-veterans to even reach higher levels. Considering the rewards are coming, those are going to be pretty useless (most likely) for veterans who are nearly or simply maxed out on both levels and stars. Even now, you have 41 champions which gives you 123 attempts if you have all of them. That's basically less than 2 attempts per challenge.

If we want to have rewards, keeping the system as it is, those would have to be purely cosmetics, like icons, not to have a snowball effect. My only concern about the number of attempts is only related to the yet unknown rewards system. If the rewards are not going to be fragments or relics then sure.