Whirly
No reason indeed why not. I think the reason why it isn't likely to happen (even if the owner of the operation did obtain that mixture of aircraft) is that you won't attract the more discerning clients needed to fly the modern aircraft unless you advertise that you have modern aircraft, and do so pretty directly. This is going to mightily upset any operators of old planes in the area (i.e. just about everybody) and in GA there often isn't a great deal of love lost between adjacent schools as it is. Also if you do advertise them explicitly you will in effect be saying that the 1970s Cessnas you have are a pile of junk...
Evo
I suspect it varies according to the area. I have a number of very active sailing friends and they say it hasn't really changed that much - unless you use an upmarket marina which caters only for the gin palaces. There are still plenty of cheaper marinas. They also point out that some French company made it a lot more popular by bringing out boats which you could move about in...
But berthing isn't directly comparable to aircraft parking. A lot of planes live out on grass, and I think most airfields have enough room outdoors. It's hangarage that's a problem if anything, and that costs the same for a new plane as for an old one - a packet. So most older planes live outdoors.
Every leisure activity will have limits; sailing's limits are the near total lack of utility (5-10kt) and the vast amount of time involved, and how much you spend on the boat. Flying is limited mainly by weather, and how much you spend on the plane and training. In sailing, no training is required and there is negligible regulation, so modern methods/equipment are adopted if they work and can be afforded, without anyone being able to stop it. In aviation there is a lot of training and it's tightly regulated, so it's easy to hold up progress almost no matter how much money people have.