The advanced features seem to be a good protection. Also the cost ist quite low, if you competing in the same market could you offer the service cheaper? I guess not. Moreover it is critical software for small businesses, I think many would pay for long term support on the original software...

Whether you can offer open source software cheaper, depends a.o. on scale.

Which is what AWS used to outprice elastic and redis.

Yes, it's called the GPL: https://en.wikipedia.org/wiki/GNU_General_Public_License

Under the GPL, a competitor can certainly take the code, rebrand it, and sell it as their own but they are required to provide the full source code of whatever "borrowed" GPL code they distribute to the end user. This ensures that the source (and whatever changes/additions are made) cannot be taken and locked up by someone else, which is possible with more permissive licenses like the BSD and MIT licenses.

Open source == no protection. You rely on the morals of your customers. At any time somebody bigger than you can take your free source, modify it, make it better because they have already a base and sell it as closed proprietary software and you can't do anything about it.

> sell it as closed proprietary software

This is not true for all open source licenses. Copyleft licenses such as the GPL and its variants prevent this.

Someone can distribute your software instead of you and thereby lock you out of any profits there, especially if they undercut your price (in the limit case they can distribute your paid software for free), but certain open source licenses prevent retroactive locking as proprietary software.

> rely on the morals of your customers ...

If they are just abiding by the license, would that be considered "immoral"?