I agree that that is a possible consequence, but it's far from guaranteed that that will happen. Although in sheer numbers many people may quit working, the actual percent of people who do could be rather low. After all, merely subsisting isn't necessarily attractive to people who already have decent jobs and can do better than one could on the basic income. It does however give them more negotiating power in terms of their payscale, given that quitting one's job will no longer be effectively a non-option for the vast majority.

This may mean that a lot of low-payscale jobs will be renegotiated, and employers who previously employed many low-paid workers would have to optimize for employing fewer higher-paid workers (possibly doing the same jobs, depending on how necessary they are, or by finding ways to automate). I don't claim any expertise in this, but I'd find it hard to believe that there isn't at least some degree to which it's merely easier to hire more people to accomplish many tasks people are currently hired for, rather than impossible to accomplish them some other way. This also is an innovation-space in which skilled jobs could pop up.

As for high-payscale jobs, I could see good arguments for any number of outcomes being likely to occur. Perhaps employers would be able to successfully argue that they should pay them less due to supplementing a basic income. Perhaps employees would balk at this and, newly empowered to walk more easily, demand that they keep the same pay, or even higher pay. The equilibrium would likely shift in some way as far as where the exact strata of pay are for different professions, and I can't claim to know how that would turn out, but it seems unlikely people would prefer to not work than to do work that gives them a higher standard of living than the basic income to some significant degree.

Similarly, people who own profitable businesses certainly wouldn't up and quit, and thus most likely any service that the market still supports would still exist as well, including obvious basic essentials that presumably would exist in any economic system, such as businesses selling food or whatever is considered essential technology in a given era. Some businesses might fail if they're unable to adapt to the new shape of the labor market, and profitability of larger businesses may go down for similar reasons, but the entry barrier for small businesses would also decrease, since any given person could feasibly devote all of their time and effort into running a business without failure carrying the risk of inability to continue living.

There would probably be a class of people who subsist on basic income, but we already have a fairly large homeless population, as well as a population of people doing jobs that could probably go away and not ruin the economy for anyone but that individual.

My point isn't that everything will turn out perfectly as expected, or that I have any definitive way of knowing, obviously, but there do exist outcomes that are good enough and probable enough to pass a basic sanity-check. The risk of economic collapse exists with or without instituting such a policy, and I'm not yet convinced that this increases the likelihood of it by a considerable margin.