Unlike any other heroes, once the collaboration event is over, the collab heroes level is fixed. Since you can't Fina them.

• Unlike the other non-7K/4L. The regular units will appear via 5/6 selector 2 weeks after their introduction event ended
• Unlike 7K/4L, you will eventually get 7K/4L to level 40; because generous event like NM has been showing us these past months. (4L selector, 7K from 7K CR points & monthly login)

My suggestion is to make them Fina-able after some time in the future. Like 2,4, or 6 weeks after their introduction?

Or if that's still considered too much, impose a limited number on how many times you can Fina them. Like once or twice only.

Basically I'm still salty to the fact that none of my 5 GGXrd heroes got to level 40, despite that I have burned over 5k rubies on them.

As non-40, they are okay for mid-game content. But in the end they fall short to the other units you can get to level 40. Not to mention it's slower to build them because you can't use Fina on them.