This depends on where you live of course, here in the Netherlands most big/well trafficked train stations have one or more secured bike parking.

The NS (Dutch National Railway) sells a secured bike storage subscription starting from 75 euros a year. And you can do it cheaper if you use your bike frequently because at some stations the first 24 hours are free.

And if you have a cheap bike that you use just for your commute you can also just put it in the free bike storage and only move it secured if you know you will be gone for >2 weeks or whatever the limit for free storage is.

Yeah well Paris is much less bike-friendly than you would think reading all the English-speaking articles about it. There are more and more bike lanes but very few dedicated parking spaces for bikes, and almost zero secured ones. There was an attempt to build some near train stations, but they are always out of order (and you need a valid subway card to access them!)

Paris is also not flat, so unless you're super in shape it's difficult to use a non-electric bikes (depending on the journey, of course), and ebikes tend to be stolen much more than regular ones.

In other French cities, I don't know enough to have an opinion; I think Strasbourg is a little bit better.

Yeah, there needs to be more secure bike parking. A reallocation of space from car parking to bike parking is an obvious solution. In Cambridge there is a bike parking facility that I would feel confident leaving a bike in for long periods but not every station has such a thing.

That being said, I know people who have done the two bikes thing, or at least one bike that "lives" at a railway station, for a long time and not aware of them having any problems.