Well, there is always the feature that says "Contains Rare Cards" to help prevent you from deleting important cards. I've deleted a few leveled 3* cards on accident from just mindlessly check marking all the boxes to sell when I was starting out. Either way, one should obviously use the "Protect" feature if the card is actually that important... In addition, it could be nice for all 4* star cards and higher to be automatically "Protected" on obtainment to help prevent the issue of accidentally deleting a good card.
That's nice that you always have something to level up, but I am quite sure that more players don't have things to strengthen compared to those who do.
P.S. what's up with your English? You take such effort in using punctuation but leave grammar mistakes
.