I don't think Kaz is trying to "BS" us, I think it's more that he legitimately can't give a solid "yes or no" answer as a consequence of how game development naturally works. There's a lot of things that can happen in development that could become factors in whether some of the older original courses make the cut or not. Some of them may've also been there to expand the track list for a title that doesn't have many licensed tracks, whereas in more recent GT games, I'm seeing a lot more licensed stuff.
I think a lot of the older original circuits would be really bad for racing online, if they were brought forward without any changes. Some would likely need more run-off areas, especially Trial Mountain. I do also really like the use of real-world locales for more of the latest courses, as Kaz mentions with Croatia and Provence. Then there were the city courses, where I think a decent amount of them could be replaced with Formula E Circuits set at identical locales - especially for NYC and Hong Kong. I really like the idea of city courses being made up of actual roads, but potentially in a fictional configuration, like with Tokyo R246. (Speaking of which, I'd love to see the actual Tokyo highways as courses, much like how they do appear as scapes, though I'm unsure if the existence of Tokyo Expressway would keep that from happening somehow.) Heck, with today's technology, I could see there being a track editor that's set at these various cities, and lets you map out your own city course around a fixed start/finish straight. This particular mode would offer less freedom than prior track creators, but those previous editors and locales could still appear, (e.g. Mt. Aso, Eifel, Toscana, and Death Valley) and by using this city course editor, some factors would become a non-concern for aspiring course designers, such as trackside scenery. I wouldn't mind the GT5 procedural track generator being present as an additional course creation mode, either.
But I digress. If we do consider online racing, I'd at least bring back Autumn Ring, Apricot Hill, and Mid-Field Raceway, but the others would likely need notable changes to keep online racing from being a complete mess. (Some would still be great for driving, though, like Circuito de la Sierra, and I'd love to see the Time Rally mode return in general.) I feel like for the most part, it's nostalgia talking instead of an actual praise of the track designs, but as I may've alluded to previously, I do think some of these original circuits have merit in returning.
I mean, I could see High Speed Ring needing some serious changes to make it better for online. And the turns at Grand Valley after that first hairpin (I recall one goes through a brief tunnel) would likely need some adjustments, too.
I think it depends on what goals PD and Kaz have for a given installment's track list. Said goals could very well make some tracks necessary to reappear, get revised, or be left behind. I don't think more is always better, especially when you consider both the player side and developer side of things. (Which is why I'm a Pokemon fan that supports GameFreak's decision regarding the Galar Dex - especially given their statements on how the "cut" species and mechanics will still return in future installments - but that's another matter.) I doubt that all, or even most original circuits will reappear in the next game, and I can't say I mind that at all, given that they were made at a different time, for a different console with different limitations, likely for a different amount of cars, and potentially with an older knowledge of track design on PD's end.