Everyone should spend their money in the way that they feel is best but that statement is just incorrect. If the first set doesn't sell, the only thing that a studio is going to think is that no one is interested and there will never be a second, third, fourth, etc. season set. Once again, it's your money and you should spend it how you want but at the same time, you should be glad that there's people that do buy the individual seasons because without them, you'd never have the opportunity to buy a series set.
that is the exact logic that the studios want you to think. and as long as they have customers who are willing to buy them season by season, that is what we will continue to get.
and one runs a much greater risk of having the studio start and not finish the project. or stall it for a long time.
lets take a look at the real picture. suppose that nobody bought season sets ? the studios would have 2 choices. one is to stop producing the dvds altogether, and make absolutely nothing.
the other is to release complete series. and that is exactly what they would do.