I planned to finish my other Zelda games on 3DS (Majora's Mask) and Wii U (Wind Waker HD + Twilight Princess HD) before starting "Breath of the Wild"... but due to too much other good games in my backlog I don't see that happening this year.

Now I couldn't resist some good Switch offers (Switch console for €277, shipping + taxes included; BotW for €48). The game arrived today and the console should arrive in the next days. 

Are there slim chances that I'll eventually finish the older Zelda games in the future or is it "Once you go BotW, you never go back!"?

Have you tried to play the older Zelda games after finishing BotW? Do you still enjoy them?

I definitely can, and I've replayed Majora's Mask on 3DS since then.
It's nice to have 2 different experiences when playing Zelda (granted MM is even quite different than all the other console ones).

As someone who hasn't played Breath of the Wild, I'd say "why wouldn't you". To put an example, I absolutely love the Resident Evil REmake, but I've gone back and replayed the original Resident Evil too from time to time. It all depends on how well you stomach archaic designs and worse graphics.

They're not even remotely similar games, so I can't imagine why anyone who did like Breath of the Wild wouldn't be able to enjoy classic (imo proper) Zelda games if they enjoyed them before.

Yeah, you definitely can. BOTW is kind of different from the rest of Zelda games, you just have different experiences, but that doesn't make them less enjoyable.

Oh yeah, you should definitely be able to enjoy them. Obviously the Zelda games between 1993 and 2013 play differently from Breath of the Wild, but that doesn't mean they're skippable. I'd argue BotW is the best Zelda game, by virtue of its emergent gameplay, improvisational combat, and first-class open world, but Zelda games like Majora's Mask or Twilight Princess have great qualities unique to them.

I find it harder to go back to open-world games after BotW, but see no problem returning to older Zelda titles.

That was my "problem" with the game. I've played Horizon: Zero Dawn after BotW, and though I really enjoyed the game, it felt really limited and constricted. Same with The Witcher 3. Both are great, don't get me wrong. But BotW open-world structure is something else. I'd love that developers learn some things about BotW and specially how a open-world must feel. 

Nah

Play a game in whatever order you want. Zelda games are mostly timeless. They are also unique enough that each game is different enough that you want to play them anyway.

I'm playing Shadow of War now, and experiencing the same thing.

Can just about go back to SS and MM remake but other than that the other 3D Zeldas with the exception of ALBW which imitates the 2D games anyway aren't as appealing after BOTW. The 2D games I'm still fine with but that's because they've always been a separate entity to the 3D games.