This is honestly a bullshit justification.
While what you are saying is 100% true.. games take time to make... that doesn't change the fact that this is at the very least showing a gross lack of foresight on MSpart.They are a platform holder and publisher. While games take time to make, their development is also planned and initiated years before their expected releases. If you look at the output of MS FP software over the last 6 years, their real problem becomes clear to see.
Its why it was just easier to buy up studios that already had projects either pending or in development.
Lets t pretend that we aren't living in the same world where Playstation and Nintendo exist. Just look at sony's output this year, last year, and even the year before that. Or the one before that...etc.
But pretty much most of the studios if not all that Ms did buy has released games already..