I'm not sure about the price/performance ratio of the actual SoCs, but if you take the Pi Zero 2 W for instance ($15), it can comfortably emulate most consoles up to the 5th generation (PS1, Saturn, N64). The performance is equivalent to that of it's direct competitor, the Miyoo Mini Plus ($80). If you want to upgrade the Miyoo Mini, you'd have to buy a whole new device, but with the PiBoy you only need to update the Pi (which will only be $15). So sure, it may still need a few upgrade cycles to match the value, but if you treat the original purchase as an investment, spending only $15 for an upgrade doesn't seem like such a bad idea. Especially when you consider the resale value - the Miyoo Minis are already cheap, so the value of an older model wouldn't be much if you were to sell it off on say eBay or something, since most folks would prefer buying a newer model which can handle emulating more recent consoles. Whereas the Raspberry Pis - even older ones sell well, one reason being a perpetual supply shortage (just look at all the people in this thread treating buying a Pi like finding unicorns), the other reason being a Pi is always useful - even the first Raspberry Pi can still be used today for things like basic home automation tasks, or even as a tiny web server, as a home security system, or wherever - hundreds of projects out there which don't really need the computing power of a current gen Pi. Since the PiBoy is targeted at DIY-ers, no doubt there would be a significant portion of it's buyers into DIY stuff and could make use of a Pi around the house. As DIY-er myself, I can never have enough Raspberry Pis, and always carry a spare Pi or two around with me when I go around - you never know when it may come in handy. For instance, when I recently visited some relatives overseas, I used one of the spare Pis in my backpack to convert their old printer into an wireless printer, so they could now print directly from their phones - and saved them from buying a whole new printer. Raspberry Pis always come in handy like that at unexpected times and places, so you can never have enough of them, but if you do, there are plenty of folks who'd be glad to take the Pi off you for their own projects.

Regarding the display on the PiBoy, I doubt you'd want to upgrade it that many times, I mean, it's just a tiny 3.5" display used to play mainly 8bit and 16bit games, getting something like a higher resolution or vibrant display would make no difference, since most of these games are low res with a low color pallete. In fact, most of the OG displays of these handhelds had shitty displays by modern standards, so any modern display in itself is a big upgrade compared to what gamers played on back then. At the most, an upgrade which may be worth could be a brighter display or an OLED panel or similar, but even then it would be a one-off upgrade and really, it doesn't add that much value when you're mainly playing retro games on there. A display upgrade for a retro console is just one of those "nice to have if you've got the spare change" kinda deal.

Also, here's the main thing: with competing handhelds like the Miyoo Mini Plus (or w/e) there's no guarantee that there will be an equivalent upgrade. They could change the form-factor in the next Miyoo Mini, or change the material of the chassis, like say switching to a metal chassis, which may make it heavy for you. For instance see the Anbernic devices, although they've been around for a while and have released many devices, a new Anbernic may not necessarily be an upgrade for your current one, with so many different form factors to choose from. Like the RG351MP, which felt like a good upgrade on paper, but in reality, most users felt it's metal body made it quite heavy, making it unsuitable for even moderate length gameplay. So the decicison to go all metal was actually a downgrade, making the earlier plastic versions better. So companies may pull stunts like this, messing around with the form factor or changing features, and then you'd need to look for some other manufacturer who may not have an equivalent, they may use Android for instance, which may result in poorer battery life and performance, so it may not be an upgrade for you.

Also, there's the problem of support - most of the official firmware on these devices are never updated by the OEM, and they also kinda suck, so most SBC gamers prefer to use a custom firmware, like OnionOS, GarlicOS, ArkOS etc. And these community built firmware may suddenly decide to drop support for older devices, for instance, see how the very popular ArkOS dropped support for the still-popular RG351M/P devices, and users were forced to look for a different firmware. Now this is where the Raspberry Pi shines - not only are there several gaming-oriented firmware you can choose from (such as Retro Pie, JelOS etc), the fact that this is basically a full-fledged computer and a Pi means there are several generic distros you could always switch to, like the official Raspbian for instance, and then just set it up to boot directly into Retroarch or Emulation Station or w/e to get a similar experience as a custom gaming distro. And that's where the Pi really shines, it's massive community support and the plethora of software (and hardware) options. A device like the Miyoo Mini may be popular now, but it'll soon be forgotten, like the poor RG351M/P, and turn into e-waste, whereas a Pi lives on practically forever, if not as a gaming SBC then maybe as something that'll turn your coffee machine into a smart coffee machine.

That's the true beauty of a Pi - it's worth cannot be simply compared in terms of dollars.