I have nothing against Overmind's answer, which is definitely a good summary of why sequence number randomisation was invented. But I'm not sure it answers the question as asked, so I will try to do so.
You are right. Nothing stops a privileged MITM from faking a TCP reset, with a valid SN, right now - randomised SNs or no. But a privileged MITM need not go to such lengths to disturb your connections through his network - he need only unplug a cable, or change a router ACL.
SN randomisation was designed to stop everyone else from doing the same thing. If your SNs can be guessed, anyone can forge that TCP reset, and desynchronise your connections. If they can't be guessed, access to the data stream is required.