Will this swell indefinitely? So in a few years it might reach up to a terabyte, which means that a lot of people will not be using Daedalus?

In long term, do you think that this will affect the decentralization of the network? I mean, when less and less people have the whole blockchain.

Yes, the chain itself will grow indefinitely. That's the idea of a blockchain. Bitcoin is now at 360 GiB or something like that. And Ethereum at a TiB? A lighter wallet by IOG (Mithril) is announced for later this year for people not comfortable with the browser-based ones. Personally, I think, the requirement to have the whole blockchain at every block producing and relay node will also go away at some point and only some archive nodes will have the whole chain.

Yes but there will be rollups and other means to optimize the size as it grows throughout time but that will continue to grow.