I did include an image in the post. The img tag and the attachment system often serve the same purpose - to add images to your post.
So what? You didn't attach anything in your post, that's the point. If you want to add images using bbcode, you can (and did) do that already. It's a bbcode, it's not complicated. The attachment system is.
The attachment system is already a core feature, which is why we're discussing possible improvements to it here. Inevitably, it will effect all IP.Board installations if IPS add this. Communities that don't use the attachment system will simply disable it. For many communities that do make extensive use of this core feature, however, streamlining the process of uploading attachments would be a welcome change. By your logic, the entire attachment system should be turned into an optional hook just because some communities don't have much use for it.
I'd be fine with making the attachment system a hook.
As has been brought up several times in this topic already, it would be possible to make the adding of a "fast reply attachment system" an ACP switch that admins can choose to toggle on and off. Browser caching will just about negate the additional overhead if it's there, and there's even the possibility of lazy-loading it.
In IPB 3.1.x, we had the option to toggle the fast-reply editor itself on a per-forum basis - who remembers that?
"Just add more settings" is the exact opposite of the direction the software is headed, and I think that's a good thing. The software should not add a bunch of stuff into it that some communities might want, but most don't, and then people just need to enable/disable a whole slew of things each time they set up a forum. That may be great for those folks who know their way around the software like the back of their hand, but it makes it so new people get frustrated with there being too many things they have to configure.
IP.Board should be designed so that it's got the set of features that will be of the most benefit to the most communities, and balanced that out with being easy enough for new admins to run, while at the same time flexible enough for power users to expand on. Uploading files while making a fast reply is something the average forum goer (across all communities, not just the ones you frequent) will rarely do. There is very little benefit to be gained from this, considering the resource needs of making the attachment system load on every topic view.