Which valuable services? I’ve had 3rd party cookies entirely disabled for a while now, and I haven’t noticed any services break, not even cross domain logins.
Maybe your services relied on the "supercookies" thus being immune to 3rd party cookies disabled :) ?
An example that I can imagine is a big onlite shopping company that has several domains, and they want a shopping cart that that works across all their domains.
A big online shopping company that moves you between several domains as part of normal use? tbh I've never seen a single site do this, so I don't feel bad saying "no, I don't think that's a legitimate reason to allow 3rd party cookies".
Besides, since they control all the domains, they can do redirects to associate your session across them easily enough. It's basically oauth at that point, which works just fine without 3rd party cookies.
not as common these days, but it's useful for services allowing people to create custom sites like blogger, where you'd want the comment form to be an iframe/embed of some sort that ties to the account of the platform, and the user-content is on a different domain.
