Nothing prevents shareholders from selling shares. However, if a company does nothing - e.g. no growth, no buybacks, the future value of their stock will decline continually. through buybacks, they can hold those share prices steady, at least. This gives shareholders liquidity as you suggest - shareholders selling without buybacks means that they
will push the price down.
There's a line of thinking which says either a) buy all your equity and go private when the innovation runs out, or b) issue bigger and bigger dividends from FCF and then eventually go out with a bang. IMO, those are strategies that should be specific to a business model. E.g. a gas pipeline may love to pay out dividends since the business model is quite steady. A big electronics manufacturer might want to buy back its shares in the hope that it can eventually go private to restructure.
my point is, the mechanic of a buyback is not inherently evil, and does provide value. I agree that the government should not be creating that liquidity. Though, in reality I don't have a specific problem with letting the government issue convertible debt if companies want a long-term counterparty. the irony is that buybacks are often seen as short-termist, but bailouts should have a long-term lens.