The issue is really availability of the vaccine at all. It would suck if a person from country A traveling in developing country B needs to get vaccinated yet again in order to move on to country C, but developing country B has a poor healthcare system that travelers would reasonably want to avoid contract with. One would then be compelled to visit a private hospital for expats at great expense, but sometimes even that option is not available.
Even if nasal sprays bought from a corner pharmacy are on the horizon, they would probably only reach developing countries years from now – and merely buying a product from a pharmacy in most countries might not get you an internationally recognized certificate of vaccination.
The entire point of such vaccinations would be to prevent another major shutdown from a new coronavirus outbreak, and thus avoid a much larger issue when trying to cross borders. The expectation is nobody is going to care much if people get the next shot anymore than they do about the annual flu vaccine because significant herd immunity will be in play.
The only way that changes is if some new and extremely deadly variant is out there, but again past vaccinations are likely still helpful. Thus, regular vaccination deployment would be a pure win for travelers.
Availability and inequality is a fair concern, but why should we stop working on something that would improve everyone's lives? Nobody said it's going to be mandatory for travel.
Even if nasal sprays bought from a corner pharmacy are on the horizon, they would probably only reach developing countries years from now – and merely buying a product from a pharmacy in most countries might not get you an internationally recognized certificate of vaccination.