I don't think I agree that social interactions are necessary to keep one sharp. In fact excessive social interactions with people that aren't all that sharp might dull one a bit.
Reading and thinking and studying can be done alone just fine.
Now as far as effectiveness in the real world, yes, social interactions and fluency is needed, but I believe this to be different from being "sharp". It probably helps keep you looking sharp though.
As someone content with being by myself, I get what you're saying but overall I don't agree. COVID was a good experiment and it busted up a lot of people.
Even excepting COVID, in the elderly the difference with social isolation can be night and day. I have witnessed firsthand one's cognitive deterioration reversed when the person moved into an assisted living community and gained a social life, and then when COVID hit and everyone was locked down the decay set in again
Reading and thinking and studying can be done alone just fine.
Now as far as effectiveness in the real world, yes, social interactions and fluency is needed, but I believe this to be different from being "sharp". It probably helps keep you looking sharp though.