Why does that matter? The only thing they need to do is correctly implement the RISC-V ISA instead of a proprietary ISA.

Because you will get into the scenario of commercial vendors basing their ISAs on RISC-V ISA with extensions not made available to the community.

Thus making the base RISC-V ISA kind of useless for software that one might actually care about.

Vendors benefit from standardization when it comes to the ISA, especially in the commodity/interchangeable market. Qualcomm is currently competing with other vendors to run the same software (Android or Windows). If they make ISA extensions which are not likely to be used by that software, then they're probably wasting their time, or those extensions were not that useful for the general community to begin with.

Useful ISA extensions are likely to remain royalty free at least, if not standardized at the level of the foundation; but if they don't, it's not the end of the world.

Except that is exactly what happens on ARM, for example.

What proprietary ARM extension are you just dying to have?

Personally I don't care about RISC V's future, was just making the point things might not turn out as the comunity hopes for.

To reply to your question, as a developer I don't care about ARM vendor specific features, because Android hides them for me.

However as a consumer, they do impact updates.