I think you're thinking too much about it.
Let's say you have a simple sub with 3 lines: 1,2 and 3
You want to insert one line after 2 so you now have 1,2,10 and 3 or 1,2,4 and 3 or 1,2,10000000 and 3
Open in aegisub, immediately export to srt without doing anything else and now you have 1,2,3 and 4
With a more concrete example where left is my edit in notepad or whatever and right is after I open and export to srt with aegisub:

Let's say you have a simple sub with 3 lines: 1,2 and 3
You want to insert one line after 2 so you now have 1,2,10 and 3 or 1,2,4 and 3 or 1,2,10000000 and 3
Open in aegisub, immediately export to srt without doing anything else and now you have 1,2,3 and 4
With a more concrete example where left is my edit in notepad or whatever and right is after I open and export to srt with aegisub:


