Will Niantic release Mew and Celebi again in the future?

There's been zero indication if they plan to allow people to obtain multiple Mythical from special research. It can be assumed at this point the one you get from A Mythical Discovery/Ripple in Time are the only Mew/Celebi you can get.

Having said that, Mew and Celebi currently are trophy-mons that have zero meta relevance. At best Mew can make a fun, different gym sweeper and can have unpredictability on its side if PvP comes out but it lacks the stats to truly excel over other dedicated specialists and generalists. I personally haven't spent any dust on mine and don't have plans to.

if you got the glitch like me
you can have two

I wonder if they’ll ever make them ex raids in the future like they’ve done with deoxys

As others have said they are not meta relevant whether you have a 78% like me or a 100%. I do not think we will get any other chance to catch another, and there is no reason to invest in one unless you really want to. 210 attack is not that great when you have better, cheaper options.

At some points they'll mass release it with its shiny form for the $$$. Maybe only during a short window, but eventually they will.

Legendaries are also supposed to be unique. Well, Mewtwo is my most common Pokemon currently. Lolled

I would not power up Mew unless it's one of your absolute favorites or you have loads of dust and RC. It's just not that useful. I maxed mine, but I'm sitting on millions of dust and had loads of extra RC back then.

I hope they won't release them again. They're mythicals, they should be one of a kind. I think Niantic will stick with that way of thinking since they made both Mew and Celebi untradeable/nontransferable.

I believe it's something unpredictable, so if it's your favorite go ahead and raise it. Mew is really cool and I'll certainly power up mine but it's a waste of RC so it's not a priority, just something that will be done one day after I'm done with meta-relevant Pokemon. I don't care if my Mew is 50% or 100% it's the same to me, I'll just use it cause I like it, not because of its performance

Doubt it, but nobody knows. Shiny forms?

On the other hand, how would you feel if you were the unlucky trainer that got a 10/10/10 Mew or Celebi? You could just say, eh, doesn't matter, not meta relevant. But if you're trying to assemble a pokedex of "living wonders" like at least a few people I know, and you got a Mew or Celebi that's under 82%, well, you're hosed. Forget about a pokedex of perfects or luckys.

As long as there's a way for Niantic to monetize it, they may do it. But will they, and when? Nobody knows.

If they think they can make a enough profit off them, they will it’s a business after all.

I wouldn't power a mew up, even if it was a 100%, its stats really arent that good.
Jirachi on the other hand will be different when released, even with MM metagross, Doom desire's still crazy strong. Assuming its not changed in somewhat before release