There is a LOT of slack in the participation rate. Those workers exist and many of them will likely move back to the workforce if the wages adjust upward to be a more lucrative job due to scarcity.
Sure, it's not going to look the same in 2050, but the economy will just run at 80% participation rate then, rather than 62% now.
There are some problems with using the LFPR, it's calculated as a percent of the civilian non-institutional population[0] (everyone over 16 that isn't locked up) for one which is very different from considering what percent of prime age[1] (25 - 54) workers are employed which is currently at around 82%. What you're saying should happen would mean that pretty much everyone who isn't dead or incapacitated would have to work, so abolish retirement and get grandma back to work at the grocery store until she drops dead.
