Other ways to get Legendaries

Don't know, but sure hope so as it is my only hope of getting one. Eggs would be nice, I would walk 100 km for Articuno!

We can only hope.

Here is a reason I have not seen mentioned...
If they want to be around for a long time, then they need to keep the little kids interested that are playing now.
My 5yo likes to play when we go to parks (at least for a little while)...but believe it or not he is not so good in raids, but he wants to fill his dex "like daddy"

There are many good reasons that Legendaries need to be accessible another way.

The accidental distribution at the very start of the game had them hatch from golden eggs, iirc. So maybe, once people stop spending real money on raid passes to catch them, they might be released that way.

Probably. It is certainly one way that Niantic could keep the game fresh. My guess is that they will make their way around one more time in raids and next year, there will be a different way of getting them.
I remember reading about a feather system buried in the code of an old update. You would be able to get them from gyms, presumably from battle performance. There was speculation it had something to do with Legendaries. It was never implemented; but, they have been talking about it for a long time. Who knows how many ways of getting Legendaries are on their drawing board?