I think that’s unfair. By tying things to domains that way, you also make it potentially difficult to change URL structure (look at how long the various go.com domains lasted across all Disney/ABC properties. I know of teams that had very real problems years later b/c the infra was still based on domain stuff that was setup 15 years prior when the parent company thought internet portals were worth spending billions on), not to mention if you want to spin-off or sell an asset. Or if you acquire an asset!

To say nothing of SSOs, where you might not want to carry your login across, but you’d like to have the option of saying “yes, I’d like to use this account here too” and then authenticate.

Also if you sign into Google, notice you bounce through YouTube for what I assume to be similar reasons

You do realize acquisitions occur?

Business operation complications are not a valid reason for the violation of the rights of humans.

Cross-domain cookies are a tool that can be used for good or bad. It's a tradeoff to block them or not block them.

So use a 301 redirect.

you do realize reorganization occur after acquisitions?