It's not about whether you or I buy a used petrol car over a new EV; it's about whether you & I buy a used petrol car over a new EV.
Emphasis on the environmental efficiency of new products in general over the environmental impact of buying new in the first place is a topic that's well worth highlighting.
Cars enter the world by being made, not by being sold. They leave the world by being damaged beyond economical repair, not by being bought.
This is not the same as party politics, even though literally every election and referendum I have voted in would’ve resulted in the same winner had I not voted. The political comparison would be voting ex-politicans off X-Factor or Strictly Come Dancing or $COUNTRYs-got-talent etc. (second hand) versus current politicians in elections (new).
That's the argument anyway. Personally, I think such coordinated effort is too hard to pull off because you are up against highly sophisticated media campaigns paid for by politicians and industry.
To put it another way: how would you react to a study which compared the environmental cost of a second-hand BEV to a brand-new ICE?
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.
Sadly, this is not the case. For most products.
Was something supposed to be different there?
Well when the government gives people $3 billion to smash their used vehicles (C.A.R.S in 2009)rather than keeping them in the used market, it does tend to cause more new vehicle purchases.
I'm not disagreeing with your point that people will eventually buy a new car if used ones aren't available, but if every vehicle was kept on the road for 25+ years, it would lower demand for new vehicles.
A fair assessment would be a new BEV versus a new petrol versus a new diesel versus a new hybrid.