That's fair, size versus capability is always a trade-off. It's also not exactly inexpensive.

I find it's just small enough, but I don't use it for every case. It's portable in the sense of being great to take with you on a plane or a trip; you wouldn't take it with you to work or school every day, I don't think. It fits in my personal item with good headphones, but there's not much space for something like a jacket. So I understand how the size excludes it from a lot of situations.

My current gaming is built around NVidia GeForceNow, Steam and the Steam Deck. I have all my bases covered and (almost!) all the games I want, "retro" and "current".

At home I have NVidia SHIELD devices on each TV. That's three rooms (one of which is on a gaming monitor with 120 Hz/HDR). I can play all my Steam (and Epic) games seamlessly on all three. They were $100 each and my founder's membership to GFN is $4.16/mo. The controllers were around $35 each as well. I don't strictly need three of them but I got them for symmetry. I like that I buy games on standard PC game stores like Steam, so if the service shuts down or gets crappy, I'll still have my games--it doesn't have the disadvantage that Stadia did. I had a PS4 and Xbone and I just never used them--I gave them away.

For the "very portable" case I have my phone. Its GFN client is great. I can use it with an onscreen controller in a pinch, or I have a Razer Kishi which is okay (standard thumbsticks and buttons). I also have a cheap Chromebook with a good GFN client I can use the play the same games. Of course, both these require good wi-fi or particularly good cell service. I've played on hotel marginal hotel wi-fi and on 4G/"5G" and that's about on the border between useable on non-useable.

Enter the Steam Deck. This covers the offline use case: planes, areas with bad wi-fi/wireless coverage. And it's still the same games with good hardware and good control. Ironically, what doesn't work well on it is GFN; but that's okay, because that's not really what I need it for.

That's the setup that works for me, and it works very well. I'm not a "hardcore" gamer and I don't own a PC at all. So for me, it is definitely portable enough for its use case.

There are various disadvantages that people might not like (besides the Steam Deck being pretty big, as you point out):

Game Selection: Obviously, this is a tradeoff with any gaming platform; not every game is available on Steam. With my setup, it's even more narrow: not every game on Steam is offered on GFN; and not every game on Steam works on the Steam Deck. There are some games I own that I can't play on the Deck; there are some games I could buy for the Deck that I couldn't play on GFN (I don't buy those games). There are still hundreds of games of every stripe available, so this doesn't faze me. It's true I can't play GTA:V, but that's probably the only game I really "miss".

Game streaming: I'm told input lag makes this not an option for "serious gamers." I'll believe them. For me, a "non-serious gamer", I detect no input lag. GFN isn't completely seamless and the fact that you're interacting with a Windows VM can be kind of clunky. No problem for a nerdy type, but there are a couple of rough edges for a consumer (for example, when launching a game you may need to interact with the Steam application in the VM to launch it, or sometimes it launches by itself; sometimes you need to invoke an onscreen keyboard and mouse input method to interact with a game launcher or close windows).

It also means your Internet access needs to be up to snuff. When it is, streaming is fine. In the past, I'd run into lag/jitter problems even when the problem wasn't on my end; that seems to have been fixed and I don't experience that anymore. I think there are also time or usage limits but I've never run into one so I'm not sure. On the other hand, the game client's connection to servers is always great (e.g. on Star Wars Battlefront II).

Limited controls: As far as I can tell, there's no GFN story for racing wheels, dance pads and other alternate controllers, at least with the SHIELD. This is probably the thing that bugs me most; I'd like to use a wheel for Euro Truck Simulator 2.

Glitches: The biggest problem on my Steam Deck is Bluetooth audio delay. I have tried remedies and system settings and they haven't worked. It's not great for a game where synced audio might be important. As it happens, my games aren't really like that and the delay is "okay". It bothers me aesthetically but doesn't make playing a game much less fun. But I'd love to see it resolved.

Another nit: cloud saves are not totally seamless, you need to pay attention to whether they're synced when using the Steam Deck. If you do, your play will be seamless; if you don't, it's possible to lose a save moving between the platforms.