The way update decimal systems work, it would not make sense to have 2.0 after 1.9. After 1.9 comes version 1.10, heres why:
Lets take version 1.8.7, for instance, and break down what each decimal means.
"1" - Shows that it is the first version of the game, the first written code. The code has not been re-written to make a new game entirely.
"8" - Shows that it is version update number 8. These updates come with new content for the user to enjoy.
"7" - Shows a minor change in the current update, usually series of bug fixes, but no new content.
Therefore after 1.9 (First game version, update number 9), will come 1.10 (First game version, update number 10), of course, with the various other bug fix update decimals after those. Version 2.0 of minecraft would be a complete re-write of the game, and would probably require the user to purchase the game again (similar to other game series i.e. Grand theft auto 1, 2, 3, 4, and 4)
Hope this makes sense