There's a moral argument I could make that suggests even with high costs lots of people would get it anyway out of a sense of civilizational fairness, or a history-of-technology argument I could make about tech getting cheaper over time, but I think the argument I like best and that works on its own is just pointing out that there's a huge economic incentive to government and society more broadly to provide such technology to everyone even for free (from their individual perspective at time of treatment).

On the production side is the incentive of having healthy citizens who don't have to stop working on account of age. There's something like 16% of the population just in the US who are older than 65, something like 20% of them are still working, and some of those not working would be more than happy to go back to working but can't because of age-related issues (the final one being death). Adding some of them back to the economy would be a nice boost worth paying a lot for.

From the spending side, currently governments subsidize medical treatment for lots of people, sometimes even for everyone and even for brand new technologies like mRNA vaccines. There's a huge network of so-called insurance providers which also play into this; some things are fully covered by a plan while others are subject to more complicated rules. The dynamics of how money actually flows around and who really pays for what are complicated but society as a whole is currently paying a lot of money for the benefit of individuals. So we can expect longevity treatments to be subsidized and affordable for some people to some extent because that's the case with every other medical treatment. We can expect it to be a large number of people, trending towards everyone, and heavily subsidized if not otherwise affordable, trending towards free (from the individual perspective), because of the nature of the 'disease' (age affects everyone, like a global pandemic) and the consequences of treating it or not. We already know the consequences of not treating aging, and a lot about their expenses. So much is spent just on end-of-life care (for US Medicare alone, 13%-25% of its budget is spent during recipients' last year of life) let alone all the age-related issues leading up to that. The government or society more broadly spending money on individuals now for otherwise unaffordable rejuvenation tech can make a lot of sense when the alternative is spending a lot more money on them not too much later for unaffordable end-of-life care.