Agreed. Prosecuting Trump in NYS would confer an embarrassment of political riches. They're just going to have to find some felonies that we're not already aware of. All of what's come out so far appears to be a matter of exclusive federal jurisdiction.
So far as a hypothetical democratic POTUS prosecuting him, I think you're right their too, when it comes to the longer term health of our democracy. But I would expect such a POTUS to be thinking about what the immediate political impact would be. They'll probably rationalize not prosecuting him by concluding that the policy issues they were elected on are more important.
Here is what I would do if I was POTUS after 2020. I would say, during the campaign, that all prosecutorial decisions will be the sole discretion of my appointed AG, that I would never interfere with the AG's discretion or even speak with the AG about individual cases at all. Then, if asked while POTUS about Trump or anyone else, I would refer the press to DoJ. Not only would that stance distance the POTUS from any decision to prosecute or not, but after Trump, it's exactly what the country needs, an independent DoJ.