[Littlemaster]

I think it is not actually needed as in some threads the 1st post may be the most relevant post, for example the giveaway thread consists number of available VPSs in 1st post, also it will make difficult for an user who visit forum less often and need to grasp the summary of threads. I t may lead to off topic as relevancy of first post may get reduced.Also it may mess up the mybb default system. As you said I will rather click on the last post if I know what is in the first post. As you are a frequent visitor I think the suggested feature may help you, but may create difficulty for others.
Anyway nice to see the suggestion.