I was wondering, like you said that jams make less sense in the world of easy refrigeration, do you think guitars make less sense when amazing MIDI keyboards are available now?

It's a good question! In general, if an instrument becomes more capable then, yes, it's going to make more sense for people to play it relative to its closest competitors. The range of sounds and textures it's possible to play on a keyboard is a big advance over what was possible on a piano, and they're also far more portable, need less maintenance, are more flexible with volume, and are cheaper. And you can face other people while you play!

On the other hand, guitar as an instrument has also gotten better. We've figured out how to make good-sounding acoustic guitars much more cheaply over time, low-end acoustics and electrics are much more playable than they used to be, and the range of what you can do with pedals has expanded. For example, an octave pedal adds a lot to a dance band where the lowest instrument is a guitar, enough that I'd guess 80% of the top guitar-lowest contra dance bands use one.

Another reason we see less substitution than you might expect is that the guitar and piano have very different strengths, and the piano's increased versatility doesn't affect that tradeoff very much. The piano is much more automated: you have much less direct control over the production of the sound. On the guitar you touch the strings, and very small differences change the feeling of your music. On the piano you give that up, but in exchange you can play many more notes at once over a larger range.

If we could see a chart of how new players, comparing guitar vs piano, my guess is we'd see a long slow decline of piano stretching back to ~1910, driven by recorded music substituting for playing at home. Then we'd see guitar rising starting in the 40s as electric amplification became mainstream and made the guitar able to hold it's own in bands. Because musicians are very conservative with instrument choice, it would take decades for the guitar to build up to where it 'should' be for it's strengths. Then, as good portable electronic keyboards became to be available in the late 80s I'd expect to see piano start to recover from its slump, though not pass guitar? If someone has data on this I'd love to see it—quickly searching I didn't find surveys, and sales records are tricky to interpret since pianos generally last longer than guitars.

This is about what people actually do, though, and perhaps you're asking what I think they should be doing? They key benefit of the keyboard to me, relative to the guitar, is how well it works as a way to get note-level information into a computer. This allows building new custom automation, and combining the note and timing information of your fingers with expression information from continuous controllers (foot pedals, breath controllers). This is minimally popular relative to standard piano (or guitar) playing, but I think is a very promising area to explore.