Eeh, that's why I said that the choice for marksmen is dictated by circumstances. Keep in mind that the regen tanks also tend to be HP-stackers, so they're guaranteed to get the full 15% bonus damage against them--is it more important to debuff their regen or to shred them faster? Against, say, a Garen, it's definitely the latter, since the passive shuts down during combat.
Likewise, I suggested that MorRem is mostly going to be built by assassins and bruisers who have to lane against a hp/regen tank: they're more likely to have sufficient health to get less benefit from Dominik's, and they're much more likely to have to fight the opponent of concern 1v1. A marksman will usually only be fighting a hp/regen tank in a skirmish or teamfight with multiple people on each side, which gives good odds of one of the following being present on their side: an assassin or bruiser with MorRem, a mage with Morello's, as well as a mage or tank with Liandry's or someone with BotRK.
Basically, a marksman would usually only need to itemize specifically to kill a regen monster if: 1. said regen monster gets fed enough to walk through the team while continuing to heal, or 2. it's a heal-centric support that they'll be laning against/that will be giving healing to other, squishier targets. This is because there are multiple other itemization paths to kill regen/hp tanks, at least one of which will almost always be followed by another player on the team. Morello's is core on a lot of mages in a lot of situations, and there's no point getting overlapping coverage of the debuff just for that first 60% of healthbar. Likewise, a lot of the preseason changes have made Ignite a much more attractive option for a lot of top and midlaners (never mind certain kill-lane supports that take it), which provides further redundancy.
The only times I can reasonably conceive of MorRem being the correct option for a marksman's armor pen is if the entire rest of the team fails to build anything against an hp/regen tank, if the entire enemy team is composed of squishies will little or no more health than the marksman, if the enemy support is Soraka, or if an enemy carry who relies heavily on lifesteal/spellvamp gets fed (as the %hp options used to kill HP stackers aren't anywhere near as effective against them). It's much more highly situational; Dominik's is pretty universal, since while not every comp has a regen tank and not every enemy player that relies on lifesteal/spellvamp/regen gets fed, just about every comp has at least one champion who builds much more health than the marksman.