I think that the idea is it matters less if it's not a disposable product.

Huh? Lego makes billions of pieces a year, disposability or not.

Why does that matter? Producing plastic is not bad. It's burning it, littering it, or throwing it away in a permanent landfills that are the bad parts.

If it's not disposable, it's not bad.

Less bad. I'm just making this up, but I assume something like this has to happen. Someone must drill/dig/mine for the oil (and maybe extract the bitumen from sand), transport that oil product to a refinery, refine the oil into something that can be used to make plastics, ship that refined product to a plastics manufacturing facility, make the raw plastic, ship that plastic off to Lego's manufacturing facility, make the Lego, package them up and ship it off to a warehouse where it will then be shipped to someone like Amazon who will then sell it and have it delivered to the buyer. There's a huge environmental costs to everything, whether it's oil based or plant based.

As it stands, I understand the research as eventually crude oil will run out. For the most part, afaik, our plastics are a useful byproduct of our oil consumption for fuel that would otherwise be more landfill waste. It's best to make reusable and long term use plastics from that material.

As to plastics from plant sources, it seems that a lot of what we do use for disposable containers and packaging are a better fit for those sources considering how well they break down. Of course, this all assumes that proper landfills are used and it isn't just shipped out of the country of use.

I'm not OP but how does that compare to disposable items like plastic forks, cups, pens, bottles or bags?

Single-use plastics are a much larger problem. Compare how many food wrappers you might see in the park vs Lego bricks.

hopefully they will move away from oil but nevertheless Lego is the only product I can think of I don't mind lasting for millennia for future peoples to enjoy