The owners can do whatever they want, it's their business. They have to weigh how much revenue advertising on uniforms and the field can bring in against possible lost revenue over the years by turning off fans. Honestly, I have no idea how many fans will watch less because of it, and it would be very hard to speculate. My instinct is that its a lot of nostalgic media types (i.e., Bob Costas) who will complain, but few actual fans will stop going to ballgames. NASCAR is wildly popular despite all the ads. Ravi's soccer jersey that says "Sega" is pretty dope; I don't even know what team it's for. I think there will be some old-timers who may turn away from the game, but a lot of young people are used to seeing advertising everywhere.
I think the key is for MLB to take it slow. If they cover the uniforms in ads, and people are outraged, then it'll be hard to undo the damage. But if they take it slow (like this Spiderman thing), people will gradually get used to the ads, and if there is a lot of backlash, MLB can reconsider when it has a better idea of the costs and benefits.