Shiny Lugia

It's not "Any Pokémon" - it's Legendary Pokémon, which are ONLY available in Raids, only available for a set - and short - period of time, and are exceptionally difficult to catch.

The reason this isn't a problem for Absol and Mawlie is that Absols are ridiculously easy to catch and Mawlie can be soloed by just about any player who puts a bit of effort in. Absol is more restricted by having to get at least 2 other players (I've done it with just 2 of us, though it wasn't comfortable), but once you beat it, catching it is simple - it might literally be the easiest Pokémon in the game to catch.

Lugia requires 5+ players to coordinate their time, then catching Lugia is very difficult (16% possibility) on top of that. AND it will only be around for a set period of time (a bit more than 2 weeks this time, most Legendaries are around for 1 month-ish).

For all other Shiny's, they have no set time frame, and other than Absol, none of them require other players to help you get it. Want a Shiny Swablu? Catch a thousand Swablus. Want a Shiny Poochena? Same deal. Want that Shiny Aron? Same deal.

So no. Legendaries are NOT "any new Pokémon" in any way, shape or form. We already get annoyed when we can't catch them, and tend to remember the ones that got away - particularly when they've got high CP. You add another reason to obsess over missing one - likely the ONLY one you will ever see - and it would cause too many problems.