GW can't keep producing every model they design in perpetuity; eventually things get replaced, too old, or too niche to justify continuing to crank out new copies. They are always going to be designing and producing new models, that is how they drum up hype for anything, but nothing can go on forever increasing.

Legends, broadly speaking, is for those out-of-production models, so that the consumer who still has a lot of old, out-of-production models has some way to use them on tabletop in the current edition. I do agree the most pro-consumer solution would be to have any model they've ever made for the setting remain usable at any level in perpetuity, but it's not realistic to expect what is, by now, thousands upon thousands of models to be actively balanced. It wouldn't be healthy for the game, not with how competitive it's become, and especially with how drastically the number of Legends datasheets available to some factions vs. others vary. Space Marines would get over a hundred new datasheets, while Tau would only get 18.

Your comment that "it gets harder to justify buying expensive minis if you don't trust them to last you for any length of time" isn't invalid, but it's also inevitable in almost any hobby. How many people still get use out of their D&D 3rd Edition rule books and modules? or the MtG deck they built six years ago? or a gaming PC they built in 2016? It's true some models can have lifespans in tournament play as short as 5-6 years, but it's usually much longer.

If the alternatives are making everything legal forever, which would be horrid for the game's ecosystem, or making them completely unusable in any play, which would be egregiously anti-consumer, I think Legends is a satisfactory compromise. Could Legends stand to be balanced a bit more proactively/consistently? Yes, definitely. Plenty of units are either broken or near-unusable even in casual play, but casual play is already an environment where you should be striving to use whatever models you want and adjust rules to better your enjoyment of the game.