I'd personally have posts not merge and make a new one, if it exceeds a certain amount of time. Say... oh... 2 hours... That seems appropriate. (GoG has 15 minutes, but meh).
Merging posts is great if you are replying to someone, then you see someone else and you reply to them, then you see a third person and reply to them. etc etc.
But if you need say 50k room in order to do a multi-piece post (say a really big review... yes i've had a 40k review before) then sometimes it's easier to pre-allocate the slots. But if posts try to merge on every attempt and the limit is 10k... kinda stuck.
Alternatively, semi-long tutorials/pieces. I did a tutorial on another site and had to do like 12 parts, which then i had the first post as a TOC pointing to each of the other parts. Took like a day to make, and thankfully didn't have anyone interrupt during that time, but just the sheer space or navigation sometimes prompts it.
No i'm not advocating changing the system, just there's exceptions to the norm. (Though having a checkbox of 'don't merge' may solve the problem. Perhaps an option if the previous message is past an amount of time or exceeds say 7k and there may not be room for the new stuff... or a BBCode as the first line [nomerge] or something that is thrown away. Well i'm sure we'll figure it out.)
