Why can't I convert a DB to a WR, or vice versa ?
and then a QB to anything else, like DB, RB or WR.

That's a very common thing in College Football to recruit a kid playing a high valued pos like QB in is High School then convert him

Converting players have very hard restrictions in this game, sadly.
I think the game would be more realistic and versatile IMHO.

Lets hope for some future version.

There is a checkbox in League Options to "allow any position change." That should do what you want. There may be a major ratings hit if you try certain changes, but the option is there.