Yes, at least Voyager was somehow not necessary, they just thought "we have the hype right now, lets milk it to the death"... which they did
between 1987 and 2001 they made over 500 episodes of Star Trek... quite insane if you think about it
In defence of Berman and his gang, they weren't alone when it came to that.
Almost every popular series have had its spin-offs or spin-off-spin-off.
Look at CSI which became very popular. Soon we had CSI Miami and CSI New York (both very good).
We had NCIS which was and still is a success. Then we got NCIS LA and NCIS New Orleans and later on we got NCIS Hawaii and NCIS Sydney.
We had Law And Order, then we got Law and Order Criminal Intent and Law And Order SVU.
And if we look at other SF-series, we had Stargate SG-1 and then we got Stargate atlantis and unfortunately also Stargate Universe.
And I won't go into Star Wars with it's numerous spin off-movies.
It seems to be inevitable that a succesful series get some spin-offs.
"Milk it to death"? Yes, definitely. But it often happens.
Personally I think that the biggest mistake Star Trek did was to come up with a retro series, in this case ENT.
It sort of crashed what we got when it come to an interesting Star Trek universe. It messed up a lot of things and never became as popular as the previous series.
What Star Trek should have done after VOY was to make a movie which would have tied together TNG, DS9 and VOY in some way, then take a break for a few years and return with something similar to PIC and maybe Star Trek NCIS, a better Star Trek Starfleet Academy than the recent one or a Star Trek Section 31.
All that when we still had almost all of the TNG, DS9 and VOY actors still alive and well.
Then Star Trek could have continued to develope from there.