I think they would be better of using an off the shelf product that has regular updates, since their current model is build channel 9, say it's done and then not update the codebase for a few years. Yes keep the main site custom (really a CMS) so they have full control over it, with the forums, when update and fixes are available then the site gets updated at minimal fuss for Microsoft, though I am not sure there are forums that mobile first (certainly not phpbb - not really sure what is flavour of the month nowadays)
In history, it took them several months after WP8 to replace the broken editor with HTML1 textbox.
And it took them another several months, after I got so frustrated and started a weekly bug status campaign, they finally add one line of most trivial HTML code to deal with newlines, string.replace(content, "newline", "br code").
And the same WP8 problem is back in W10M without any fix. They only fixed the insane infinite loop crash on W10M, but, the same cursor problem is still there. And they will NOT fix it yet again.
As of now, I no longer has the energy to advocate excellence. I only have the energy to LMOA when their broken forum affects other users.