The masses decide what 'Bitcoin' is by the adoption and usage exactly how they decide what their search engine or web browser is.
With SW as a SF BTC is a fork as well and it might be the NetscapeAltavista for the masses.
It still follows the same chain though is didn't fork off you decide it is not forced on you if you don't trust or want to use segwit then you don't have to you decide by the type of address you use
If you carry on using legacy addresses then you are using the exact same Bitcoin as you were several years ago nothing changed in that respect the data is still stored in the block in the exact same way as before.
The only time the data structure changes is when you use a segwit enabled address which in laymens terms just moves the signature data from the transaction field to a separate merkle tree that's pretty much it