Let's see what proxmox staff will say. In my opinion it looks like they decided to keep the feature and new implementations low and instead focus on implementing core upgrades like kernel ect like you see in the
https://pmg.proxmox.com/wiki/index.php/Roadmap#Roadmap
Adding new features always requires a lot of resources and will make your product more complex and harder to develop in the future if every enhancement is not implemented really well. You also need to take time to validate any core changes to the product or you will have a lot of issues this also takes a lot of time and effort to do well... I don't know how big the core proxmox mail gateway team is but it does not look very big. To me it looks like PVE is the main focus for proxmox and they focus their work there more than on mail gateway.
As every company you are limited by your resources how much it's available for some product and how skilled are your employees. If you want to make a real change to the product you should try to join the company and try to make the changes from inside it's much harder to do that from outside. I also don't know how many subscriptions does pmg have and if it's enough to cover the development of it to me it looks like they make it easy for people to use without subscription if you don't care about regular upgrades you can run with it they could also change this. With more subscription's you can get maybe enough to add some additional developer to the team that could focus on implementing new features. Good developers are hard to come by and will cost a lot.
I have the same thought as you trying to keep my configuration as simple as possible but this is just not possible in some cases if you want to offer good mail filtering services to the companies you are supporting. PMG requires a lot of modifications as default configuration will not be very effective for filtering I am guessing by reading the forums the team decided this route because a lot of inexperienced users use pmg and if they make it more aggressive by default it will be even harder to use for new people.
We used to have before 2 barracuda virtual machines was super easy to upgrade and worked well out of the box (primarily just 1 in use another in standby) for filtering all email incoming and outgoing now we have 3 server for incoming (main pmg, eset icap, quarantine proxy) for outgoing 4 (proxy, primary pmg , secondary pmg (dkim singing), standby) that are highly customized because pmg does not support authentication so it's much harder to upgrade. Also no filtering for outgoing... I was not the one to configure most of this but if I was and I understood the limitations of pmg at the time I would not select this product for our use case it's just does not make sense. It's not a multi tenant solution and will require a lot of customization to make it work well. If you are using it for one company only and have someone who is willing to work with it then you can use it no problem. GFI MailEssentials looks much better for multi tenant use we might even switch as they want to give us NFR license if we use it so we will see.
This was a bit off topic but just so you get an idea I think we are losing money by using pmg as it requires so much time and effort daily to keep the mail filtering at high level (even with manual adjustments still many phishing mails will go pass filtering) and to keep all the systems up to date with all the modifications we want but that is mostly a problem with us selecting a product that is not appropriate for our environment just because it looked like a "free" solution but if you look in more detail you will find the hidden costs like with everything. I had a hard time convincing my boss to cover even the basic community subscription because they want everything "free" without understanding true costs with every solution.
