It's acceptable ONLY if they are not day one games but have been in the market for some time. Sales become negligible when the game ages. 

Panzer Dragoon Orta for XBox includes a full port of the original game.
https://www.ebay.com/p/Panzer-Dragoon-Orta-Microsoft-Xbox-2003/9179
Probably even cheaper if you look around in second hand stores.
It's on the wishlist at GoG.com as well so it might end up there eventually.

Plus there's always the option of not playing it. There's plenty stuff that is affordable.

If the game is no longer sold by itself, emulating is fair game. No exceptions, especially when the alternative is also rare. 

Pirating is not OK.

By emulating, do you mean pirating in this case? You still need to get the game first to emulate it.
What game is so rare you have to pirate it?

Or do you mean if you have the game you should wait until it's no longer sold before running it in an emulator?

Some games end up becoming relics. They are rare and expensive. Some are outright no longer sold. Those are alright. 

Alright to emulate, sure. Alright to pirate, no. Alright to download if released into the public domain or as freeware, yes. There's abandonware, which is basically a nice way of saying pirate away as the copyright owner is no longer able to or won't do anything about it. Morally it's alright, legally it's not.

For really old games it's a good thing anyway. I doubt many of my old 3.5" disks still work, 5.25" floppies are definitely unreadable now, cassette tapes with msx and c64 games are done for. Yet people weren't obsessed with copying books and movies out of for fear of them getting lost. Why is there no faith in creators of software to look after their stuff that we have to make lots of pirated copies :)

I was indeed referring to abandonware, which most games from the fifth generation and below are. Legally it's also alright, as though there are laws in place to protect the piracy of content, use of a product which is no longer in production falls under fair use. 

Afaik fair use has more to do with using part of copyrighted material (referencing) for your own work, for example let's play videos. I'm not aware on how that's different for material that's out of print. Legally it's 70 years or so after the product was made before it enters public domain.

Morally abandonware is considered fair to use. Legally, downloading abandonware is still considered piracy, yet no one will sue.
http://www.makeuseof.com/tag/what-is-abandonware-and-is-it-legal/

That's not Emulation at all.

It's a mod plain and simple.

It is no different then Morrowblivion or Skyblivion or Skywind etc'.

The age of a game doesn't change it's legality at all.

Zelda: Breath of the Wild is perfectly legal to emulate, provided you purchased the original game.