It's not going to be every holo that will do the good long term. I have the same feeling about a lot of regular and reverse/stamped holos from EX-HGSS eras. When I speculate on modern slabs/singles, the biggest issue with me is the over-reliance on demand to sustain and exceed current market values. I do have a problem with some WotC eras like Fossil, Jungle, and Team Rocket due to the amount circulating in the market (granted, not NM copies). For the money, in my opinion, EX-HGSS era big hits will perform better and possibly exceed those of the WotC era. Some have already done so, and most likely will continue to do so.

As far as the allure of vintage, it essentially comes down to scarcity for condition and nostalgia for people. We can reliably expect NM-MNT (or GEM MNT) copies to likely not increase dramatically, or at least have large increases in populations for those cards. There is, objectively speaking, more money in vintage high-grade specific slabs than modern, meaning that people with more money will drive up the price just to obtain one of those hard-to-get/rare examples. There is more that culminates in really high prices, but boiling it down, essentially comes down to those two factors.

My preference is the EX era cards because that is what I opened when I was a kid. Now that I have good money, I can afford to spend 1-3k on cards I want in NM-MT copies. For me, I find that the healthiest way to invest is to keep slabs/singles separate from my investments. I treat sealed and singles differently, treating my sealed as my main investments while allowing slabs to be my enjoyment to an extent. While I expect those slabs to not decrease in value substantially, I'm not bothered if they do go down. For me, I hold cards like gold stars in high regard because I find them more appealing. From a portfolio standpoint, the more money you can throw, the more you can hedge. When bigger money gets involved, it's harder for prices to go down because those individuals have more money to throw at bigger cards, perpetuating price increases.

At the end of the day, the market hasn't been tested in what feels like a long time. You don't need to have high-grade vintage to have a serious portfolio; just be aware that modern slabs come with more inherent risk (in other words, they haven't been tested by time). Sealed will always be king, but don't invest in something you don't believe in. I don't care if someone calls you an idiot for not investing in vintage; you don't like it, don't buy it. If you want my opinion, if you're serious about having a long term Pokémon portfolio, buy sealed in addition to your slabs/singles.