Just charge for support, or if that is too harsh. If that is too harsh, charge for upgrades (but give point/minor bug fixes for the version they have for free).

No support contract? Pound sand.

This sounds good, but in the real world it leads to massively upset customers.

The problem exists from both sides of the coin. Firstly the bulk of customers don't purchase a support contract. So there is very little income to pay staff. So the "support" department has very few people. They're also not very good because low wages means staff turnover.

Then Betty phones with a problem. Significant time is spent explaining to Betty that we can't help her because she (or more accurately her company) doesn't have a contract. She's fighting back because an annual contract seems a lot for this piddly question. Plus to procure the contract will take days (or weeks or months) on her side. And it's not I any budget, making things harder. Betty is very unhappy.

The junior tech doesn't want to be an arsehole and it's a trivial question, and is stuck in the middle.

We switched to a SaaS model in 2011. Users fell over themselves thanking us. They don't have to justify it to procurement. The amount can be budgeted for. No sudden upgrade or support fees. Users get support when they need it. The support department is funded and pays well, resulting in low staff turnover, and consequently better service.

Plus, new sales can stop tomorrow and service continues. Funding for support remains even if sales saturate the market.

Consumers may dislike SaaS, but for business, it absolutely matches their model, provides predictability, and allows for great service, which results in happy Users.