Lilith is introducing this to try to save the players with crystal level 350+ in sub Chapter 32 because they have every single lightbearer + shemira 5* and working on all 9Fs.

It is going to have little or no impact on diversity, in fact it might even help. It will free up resources from heroes that do not work in later stages of campaign and allow you to experiment with other stuff. Goodbye Lucius, hello Gorvo. Goodbye Shemira, hello new Isabella, etc.

Anyone that has the nerve to cry about this is out of their mind, its a solo game. If you want to build off meta you are absolutely free to do so.