A 75 player with an event shell that was active when ToAU was in its prime (for like 5 years) could pretty reasonably expect to get all the requirements for a mythic (except Alexandrite) done with a minimum of effort.

At 90, most of the requirements are outdated and not done as much anymore. However, the extra 15 levels make things a lot easier than they were at 75.
* Einherjar Tier 1 and 2 can be pretty consistently completed by a single party for about 2k Ichor.
* Nyzul Isle can be farmed by three players that are likely to get about 6-15 floors (~2.5k Tokens per tag average assuming you're helping someone climb from 1 to 100).
* Beastkings still come with adds, but I was in a pickup alliance last night that took all three out in ~4 hours. A single well-organized party could probably take them out.
* Most assaults have always been trio-able, and even more are trioable at 90. It comes down to having two friends. I got captain at 75, and I look forward to doing all the assaults again at 90. I've always enjoyed assaults.
* Alexandrite is an eternal sticking point, and SE has pretty much sworn this won't be changing.