Swifties should have tenacity + slow resist, and Mercs should have something vs spells

Hear me out.

Swifties and Mercs serve the same role, just against different opponents. You never buy Mercs for the MR (or you shouldn't, because other MR items are better) which is the opposite of Tabi. Tabi is better than other armor items against autoattacks for the gold cost. This makes sense because it prevents you from buying a different boot, while other armor items don't prevent you from buying any other armor item.

Situational utility can be a reason to pick another boot, but I think with Mercs and Swifties it's too situational. You should be able to take both. I think that would be more competitive.

Like imagine you're against a team with two strong autoattackers (like Yas and Draven maybe) but they also have a champ that swifties is good against, and another champ that mercs is good against. Well, tabi is good against two of their champs (and this scenario is common since a lot of champs are autoattackers) but mercs and swifties are each only good against one enemy in this scenario. You are almost forced to take tabi because reducing the slow or CC from one guy probably won't make any difference for you in teamfights. But if you could have both then it would be a more competitive choice in this type of scenario.

And then ofc if Swifties is the anti-CC boot, Mercs can do something vs spell damage. This would make a lot more sense because tenacity isn't necessarily useful against mages. While they do tend to have hard CC, they don't need the full duration to land their combo anyway. It's pretty good against catchers because it reduces the window of time that their team has to follow up, but not all catchers deal magic damage. Similarly, not all champs with hard CC deal magic damage.