I see... but isn't that a bit of a mess? If I want to run a full node, now I need to run 2 different folders with 2 different software with 2 different clients just to go between formats?
Wouldn't it be better to have it all inside the same software and be able to select what you want or maybe that would make newbies too confused and it's better that newbies use segwit by default (since they don't know anything about this anyway and sometimes excess of options make newbies stressed)? But then again, advanced users like myself would suffer from having to run 2 softwares..
No, you do not need to run two different softwares with their own datadir. Bitcoin Core has functionality now to have multiple wallets loaded at the same time. This is known as multiwallet. So how it would work is that you create one wallet which is set to only ever make segwit addresses, and another wallet which is set to only ever make non-segwit addresses. When you want to send, you choose the wallet you want to send from. When you want to receive Bitcoin, you choose the wallet you want to receive to and you will get an appropriate address. Note that this is all still in development and still in the design phase, so this could change and not be the actual final design.