Yep, it was definitely meant to be the last one, even if the plot was the exact same one as Revenge of the Fallen. I even remember the TFWiki page for Dark of the Moon ending with the typical "Til All Are One" quote like they do with the final episodes for each series. Although the original script (As well as the comics and novelization) saw Megatron calling a truce and being spared by Prime instead of having his head ripped off, which tells me they didn't plan this to be the end, but decided upon it later on. And honestly, I'm glad it didn't end there. The trend of having 3 movies then reboot kills all sorts of creative freedom from my point of view, so I was happy for a continuation going off in a slightly different direction. I'm trying to figure out how the hell a pure reboot would have worked. Would it have been different CGI animation? Would they replace Peter Cullen for Optimus or keep him on? How would they do the introduction differently? Would it feel too similar to the 2007 film or would they attempt a straight up adaptation of G1? I feel like they wanted to reboot it, but for the life of them couldn't come up with an original means of doing so.