Alejandro, sorry to hear this but do keep going! The fact that you have a user base already is fantastic.
In my career I have met many people like this, some people are just difficult and if they are already being like this imagine what they will be like when they have invested money and are more involved.
You didnt mention if the app is mobile, desktop, web etc. Can you clarify?
For mobile I'd say the winning argument is that B4X always creates native apps. For desktop its a bit harder to defend as Java apps just dont look great
For web/server the beauty of B4X is that you can pretty much use any front end look at all the examples using material, skeleton, etc. The backend shouldnt really matter as long as good practises are in place.
I come across many technologies in my day to day job. Nothing is perfect. Everyone love things like node.js right now but to be honest node is an absolute hog for system resources. Many of my B4X server applications run on 1 cpu and 500mb RAM. They run on linux, windows and mac. I see that as a much better solution.
Some shiny new tools are not always what they seem. Depending on how long youve been doing this you may or may not know but there was a once a shiny new tool called AppForge which allowed you to create Pocket PC mobile apps in Visual Studio. Sounds great right? It was great but the company became insolvent and as a result they turned off their activation servers which left thousands of developers unable to continue developing - see here
https://en.wikipedia.org/wiki/AppForge
This kind of situation will never happen with B4X. Even if Erel stops development of the IDE the users can still create new libraries to extend functionality. If Erel turns off the B4I build servers you can use a local server to build. So you arent locked in.
Just a thought but perhaps we should collectively create a Q&A for these kinds of scenarios?
Good luck!
PS I just did a Google for Pocket PC ide and the first result mentions Basic4PPC (Erels product).