For tools that depends on network effects to provide value (such as P2P), the on-boarding process and learning curve are quite important to reaching the required levels of adoption.
This is especially true when you are competing uphill against alternate tools with established networks that provide more network effect value.
Yet all the examples I gave (rotary dial phones, 8 track players, VCRs) had very high rates of adoption. But I'm sure that there were people from generations once or twice removed from the introduction of such technologies that never really learned how to use them.
None of those products are close to as dependent on network effects to provide value. Rotary telephone didn't depend on other rotary telephone users, just the existence of automatic exchanges. 8 track and VCR didn't depend on other users of the format, but on the availability of content in that format. While they all gain advantages in pricing / availability from networks effects, the core functionality of these products does not depend directly on the network.
The rotary dial phone had no real competition.
8 track player adoption was largely driven by large automakers offering 8 track players as factory and dealer installed options (and thus simplifying the on-boarding process). Once 8-track's portable niche was taken over by the easier to use cassette tapes, it declined fairly quickly.
VCRs did not face an 'uphill' competition against a competitor with an existing network effect advantage. It can even be argued that Betamax lost (despite having a product that was percieved to be better) because its higher cost relative to VCRs hampered its on-boarding process.
There will (almost) always be people willing to cope with a worse on-boarding process and/or steep learning curve. (This is far more true for a product that has functionality not otherwise available.)
However, if your product is dependent on network effects and you are competing uphill against a similar product with a well established network, then your on-boarding process and learning curve will be critical (but not the only factor to) your success.
It is hard to find a group of products that is more dependent on network effects than P2P software (besides centralized messaging platforms).
It seems that the grandma/grandpa use case always comes up in technical discussions. I've never really seen the need to cater to those who aren't willing to learn how to use the tools they have.
Could a person born after 2000 figure out how to use a VCR? Set the timer so that it records the correct program? What about a rotary dial phone? Do they know how to make a collect call? What about directory assistance? Could they figure out how to use an 8 track player?
They probably wouldn't, but the motivated ones would avail themselves of the resources at hand and figure it out.
The same thing applies to current technology.