I like the idea of using roughly the top 30 pitchers as the criteria for considering someone as an "ace".
There are, in theory, 150 slots in MLB's 30 team rotations. Being in the top 20% makes you part of the conversation.
Porcello would clearly be the best pitcher on several teams right now. Does that make him an ace? I wouldn't go that far, because then any pitcher that is better than the worst #1 pitcher on a given team could be called the ace of that team, if he was traded there.
I know the criteria is hotly debated, but being 18th in WAR is pretty impressive. 24th in WHIP adds to the argument that Porcello is at least close to an ace.
Personally, I'd call him a decent #2 or a solid #3.
I'd call Sale the best of the #1's. The ace of aces.
I'd call Price a decent ace or a solid #2.
Eovaldi can be a #2, if healthy.
ERod may someday be a #1 or 2, but I'd call him a decnt #3 right now.
Severino is a true ace.
Paxton could be an ace, if he ever gets near 200 IP. He's a 2, now.
Happ has pitched like a solid #2 over the last 3 years.
Tanaka is hard to call. I'd say decent #2 or solid #3.
CC is on his way out, but he's likely a good 4.