Partly this has been because I did some traveling (a lot of traveling; fulltime for four years, in a motorhome). I spent a lot of time in places that aren't Silicon Valley or other thriving metropolis. Detroit, Slab City, New Orleans, a variety of rural places. There are people who have been completely pushed out of the legitimate economy or merely scrape by at the very bottom, for a variety of reasons. And, it's not a small number of people. It is huge swaths of the population (as much as 39%, based on the poverty line, but more or less, depending on how you define it). That's not sustainable, ethical, or efficient.
The CEO to worker pay gap has also contributed to my changing view on this. When a full-time (or part-time who holds another job) Walmart worker doesn't earn enough to rent a tiny apartment and pay for basic expenses, while the CEO makes orders of magnitude more, it becomes apparent there needs to be other ways to push the income down. If corporations won't behave ethically by choice, there needs to be outside pressure. Increasing efficiency and increasing revenue has not resulted in improved quality of life for workers; and that is mostly reflected across the board. It is not merely egregious examples like Walmart. Even many "good" companies have horrible pay gaps and little loyalty to their employees (while expecting loyalty and dedication in return).
And, that doesn't really even begin to address the changing nature of work. Many skilled work roles, those jobs that the American middle class was built on, have disappeared from the US economy in my lifetime, and this hasn't been reflected in a subsequent increase in pay or benefits for lower-skilled labor jobs that replaced them (and some of those jobs were not replaced...they just don't exist anymore). All while real estate prices and rents have gone through the roof. So, again, large swaths of the population, the kind of people who could have been middle class home owners in previous generations with reasonable retirement savings, are now renters who live paycheck to paycheck, or worse, live on credit. There's certainly room to talk about why the real estate market is as lopsided as it is. And, there may be room to talk about increasing skills in people who currently work low-skill jobs (though, evidence indicates there are many categories of job that are simply not going to exist in the future). Again, it's not sustainable, ethical, or efficient to have so many people living in poverty.
Finally, we already spend a couple trillion dollars on welfare programs. The increased cost for extending that type of benefit to everyone, while removing the bureaucracy of maintaining the existing programs (eligibility compliance and case workers, etc.), is actually not as dramatic as it first seems.