Depends on what the study is trying to prove. You seem to be angling for some sense of "fairness", as if this is a game where EV / ICE "compete" and we want the rules to give each side equal chance or something like that. This isn't sport; if you're doing a scientific study you only care about getting the correct answer to your question.

If your question is whether a second-hand BEV has higher / lower environmental cost, why would there be a problem doing such a study.

It's not a foregone conclusion either; e.g. (hypothetically) a lifecycle analysis could show an EV to be less optimised for long-term use (spare parts environmental cost, etc.), compared to a new ICE with a more sustainable long-term maintenance story. This might be unlikely but you can't find answers to these questions without such studies.