I agree with many of the developers who have already chimed in here.

The only thing really to be gained by charging solely for support is the perceived idea that you don't support your products because that is exactly what will be claimed when a user has a conflict. People paying for support is a rarity. It happens on occasion but they are the exceptions not the norm. Like Carl said above, you can do this but you won't be able to build a company like Gravity. You would just have a small shop that plugs holes instead of building ships.

If your desire is to patch holes that's great. There is a market for you to do exactly that. I just don't think that's why most of us are developers. Some of us are building full application level products.

But let me try and answer some of your questions directly.

How would this change the way vendors today do business?

It would put most developers out of business as it wouldn't be able to sustain the full-time attention of one developer much less a team. Sites that currently redistribute the work of others would cease to exist because that work would slowly disappear. If no one in this market is willing to pay the developers most of them will no longer be able to develop for this market.

If the real value is in support and upgrades, then the question I have for commercial plugin and theme developers is why are they charging for their GPL licensed code?

That is not where the real value is.That is where some of the value is. Much of the value is on the hundreds and for some of us thousands of hours of time we have put into building and improving our products. Much of the value is in knowing we will be around to continue that development far into the future. And many are charging for GPL code because of my answer to your other question above.

In the end the GPL license doesn't enter into our decision to charge for access to our code. We charge because the work that we have put into that code is valuable. If someone else doesn't see that value then chances are they were never going to be our customer to begin with.