tcp – What prevents this specific type of attack from being viable?

Imagine a user has an ip of 1.2.3.4

The server the user intends to connect to has an ip of 2.3.4.5

An attacker has a machine with a promiscuous network card on the user’s local network.

The attacker also has a server on a seperate network with ip 3.4.5.6

The user sends a request to 2.3.4.5, which the attacker had DDOS’d. As such, 2.3.4.5 will not respond.

The attacker’s machine on the user’s local network sniffs the request and sends it to the 3.4.5.6; 3.4.5.6 is set up to take this information to form a request to 1.2.3.4 where it spoofs the IP of 2.3.4.5 and has all the required TCP Sequencing information to form a request that looks real.

When the user sends another request, it is once again sniffed by the attacker’s local machine and sent to 3.4.5.6 which can then send another false request. The cycle continues.

Since 3.4.5.6 appears to be 2.3.4.5 and since 3.4.5.6 is NOT located on the user’s local network, the user’s firewall is unable to detect any foul play.

I’m assuming that this type of attack is not actually possible and that somewhere there is a misconception on my part about how networking works. Why would an attack like this not be possible?