SACKs have been in TCP for 25-30 years now (Widely adopted as part of New Reno, although RFC 2018 proposed the TCP option and implementation back in 1996).

That said, the typical reason why TCP doesn't send packet N+1 is because its congestion window is full.

There is a related problem known as head-of-line blocking where the application won't receive packet N+1 from the kernel until packet N has been received, as a consequence of TCP delivering that in-order stream of bytes.

Sorry, I didn't literally mean n+1. What I mean is probably better described by 'head of line blocking'.

Basically, when transmitting a file, in principle you could just keep sending n+k, even if the n-th package hasn't been received or has been dropped. No matter how large k is.

You can take your sweet time fixing the missing packages in the middle, as long as the overall file transfer doesn't get delayed.