The potential problem is not so much the price, which is as you say very reasonable, but getting locked into a proprietary product that you are then attached to. What if they stop developing it? What if they take it in a direction you don't want? What if they move to a higher and higher price? All things to consider.
As for me, you can pry my Emacs from my cold, dead fingers.
The answer to all of the above is: 'Don't upgrade'. As it stands ST is an excellent editor with a rich plugin system around it, even if the dev decides to call it a day it's unlikely it's going to stop working in the next 2-5 years, and additional features can be introduced via the plugin system.
I do understand the concern though, but for a lot of people (myself included) ST is a better solution than a non-proprietary solution.
A waste to who? It's the editor of choice for a large number of people. It provides value to developers. In turn, the people who spend all day building it (I assume from a comment above) get paid to do so. Do you want to work for free?
I may be in the minority, but I don't think all code should be open. The people behind Sublime have made a great product and continue to improve it. Do you think they'd receive even half of their income if they made the code available?
Step 1. Remove nag message.
Step 2. Release to the world.
Day 3. Tonnes of people jump to non-nag version and never pay.
The plugin system and API is available for those that wish to complement or extend the editor. Making the code available directly harms their ability to make money and their ability to continue to invest time and money building the product.
As for me, you can pry my Emacs from my cold, dead fingers.