TL;DR can't be replaced with just "Summary", because it carries much more meaning than that. It's a preemptive retort to any criticism that the original text is too long. You need to understand the full evolution of the term before dismissing its value: It originated from commenters telling an author the text is too long or needs better formatting. Its use as a summary prefix acknowledges and sidesteps this issue.

Rather than simply saying, "Here's a short summary," TL;DR says, "I know this is long and you probably hate me for it, so here's a short summary to mitigate your unhappiness."