I'll rephrase your words:
"I didn't make myself clear that I was speaking retrospectively, and while they haven't always been split, they were already known to be ssp. that were clearly fenotypically different so better to be safe than sorry".
With regards to the Treepie, the question whether you would have gone when it wasn't known (pre-1991) / people weren't going there (I don't know since when Kaeng Krachan was firmly on the birder's map) is irrelevant as you can't pose the question if the treepie is worth going to a place if it either wasn't known, or if people didn't go there; It's relevant if people go there since the Treepie was discovered there / since it's on the birding map, and the question that has to be answered if there are good reasons to search for that bird.
The answer to that question is imho less black-white than you suggest: If you don't go looking for the Treepie in Kaeng Krachan because it's easier elsewhere, you still have to see it elsewhere, and world birding is a slow marathon, so it could take another decade before you're finally in Treepie territory somewhere in the 'easier' place in Vietnam, for example. For many people, Thailand is a destination they go first, and only after Thailand they will visit Vietnam. It's always a good thing to reduce the number of target species in Vietnam having seen many of them in Thailand, so in the particular case of the Treepie, given there aren't many hard targets in the upper elevation section of Kaeng Krachan, I would include the Treepie in the 'put in some effort' category, without compromising on searching for e.g. Rusty-cheeked Hornbill, but in hindsight: I would even consider Ratchet-tailed Treepie in the top 5 of best birds you can see in Kaeng Krachan higher elevations (next to the Hornbill and Pin-tailed Parrotfinch, with the parrotfinch not exactly a site-specific hard target either, maybe some other species but can't really think of 'hard' targets over there). I haven't birded thoroughly in Thailand but I only have about 5 birds I can realistically target in Kaeng Krachan and none of them is a bird I have to see in that park as they're widespread (boogie) birds.
The other (in case of the Treepie probably less relevant, but still, not fully known) reason is that many songbirds have different ssp. across their range and some of these ssp. are split candidates. Even without splits, I have, almost a decade ago, realized I shouldn't skip non-lifers from the want-to-see list on my overseas trips; these are not target birds sensu stricto, but it's better to be safe than sorry, and it's simply a good thing to see birds over and over again in different places. So nowadays, I strive to see everything everywhere, ofcourse without compromising time looking for targets. When I'll be in Vietnam, I'll be looking (or more likely listening) for Ratchet-tailed Treepies.
What DMW says about wetlands and coastal sites makes much more sense with regards to giving examples, as most (not all!) shorebirds are monotypic / migrating birds that are nice for the list but 100% irrelevant as a target bird. Examples as Ratchet-tailed Treepie, but especially Limestone Wren Babbler are, imho, a bit unfortunate if one wants to explain how world birders manage targets.