You need to consider the following at least:
- traffic implications to your forum when migrating / search engines
- your existing user base are cool with Discourse and features like infinte scroll
- adrevenue, do you depend on this, will it be effected?
So you might have the technical point on side, but one or all of these or an x-factor unique to you (the loss of some custom dev/feature that keeps people returning) or both of these may hamstring your new platform post migration, a user exodus or revolt will neutralise any technical gains.
Discourse is vastly superior to phpBB on many levels, for example the ease of finding information, which can often revive old forums content by being more discoverable, but if your users revolt all your hard works hopes and dreams will go up in smoke!
Depending on the size and traffic you need to carefully think about users reaction to a change. You can try bring them along in whatever manner you might achieve. This may smooth over any problems.
Generally, people do not like change.
You can’t please all of the people all of the time, only som eo the people some of the time etc. etc.
If you don’t give a dam about any of the above, then plough ahead and best of luck. 