The problem isn't providing replacement parts. I think the issue is that locking their customers in their maintenance services is part of their business model.
Most likely the lifetime of service (decades) wouldn't easily be fitted into the margin of a specific model.
Essentially, you can't just look at the one product in isolation and think "this should exist from a competitor", as if anyone could easily make a company to go out and do it... you instead have to consider the ecosystem of companies and the numerous barriers to entry (such as patents and limited supply lines and the enormous R&D costs) and what the incentives in the market are, and it turns out that, if your product is one where the competition is already scarce--Apple + Samsung make up 80% of the mobile phone market, with the rest likely targeting people who don't have enough money to pay for high-end phones; there is a similar thing going on with tractors, with five companies controlling >70% of the market, and AFAIK mostly now selling locked down machines--you can't just expect to magically find a competitor pop up that is going to fill this particular interest. If you manage to circumnavigate all of the hurdles to build a phone that works as well as an Apple or a Samsung device, you are almost certainly going to join the oligopoly and lock your device down also, as the incentives are just too poor otherwise. The only fix for this is regulation and, to the extent to which regulation actually already exists here, legal precedent.