I think you’re forgetting where Linux was the most successful by far: Servers and Android. Server guys do what they want, if you tell them they can only use software you allow them to, they will laugh at you and buy their data center elsewhere. Android has had locked bootloaders forever (I actually think even my very first phone had one).

So maybe development would have been harder? I mean, we don’t have looked bootloaders on desktop even today, not really locked at least, so it’s hard to tell. Linux’s main audience would not have cared I think.