3CX already uses "Let's Encrypt". How much harder would it be to let the 3CX implementor provide the desired FQDN and automate the whole process?
I have no doubt this boils down to priorities. 3CX already provides a functional solution provided you use one of their subdomains, so Nick more than likely feels 3CX responsibility is covered and can move to other priorities. As you know, if you want to use your own FQDN and Let's Encrypt, you can do it yourself using certbot. We (VoIPTools) get suggestions from customers multiple times every day, and as a business owner / developer I have to filter through many, many good ideas and focus on what is most important.
Perhaps what 3CX could do as a compromise is provide a guide for setting up Let's Encrypt using certbot. It is not difficult to do, but I have to google the process every time.