Unfortunately software development costs money and due to payment processing costs it becomes unrealistic to charge less than a few $ unless you are a massive company like Apple who can get much better rates and merge transactions in many cases.
Taking WindowBlinds for example, it will need some specific work to support the new Windows 10 features. To give this away for nothing means we would have to had charged more originally for WB8 which means those who have no interest in Windows 10 would have to have paid for it. You will notice we update our applications and using Start8 as an example here have added numerous features based on customer requests without a single additional charge.
As you say there is nothing requiring you to upgrade and we are not stopping Start8 or WB8 running on Windows 10(Well I think WB8 installer may block you now due to issues from the OS, but you can force it), but obviously they are not officially supported on Windows 10 as they were designed for Windows 8 (or Windows 7/8 in the case of WB8). I imagine we will have a new WB for Windows 10, though I don't think this has been officially decided yet and the upgrade fee may well be 50% off as it cost more initially.