Don't you have an impression, that few years ahead there will be only very few blockchains out there?

This is a very valid point.
The comparison you make with email companies is very interesting, perhaps a similar phenomenon occurs with blockchain, although we must remember that blockchain has a unique and disruptive character that may not behave like its predecessors of the classic internet.

A hypothetical monopolization of the new era of Internet with Web 3.0 and all crypto technology would be subject to other investment standards due to its fully distributed nature.

Time will tell.