An interesting and valid point. I see two possible issues, one technical and one societal. Making links migrate with account movement might be workable at first. ( say one migration) but as time passes and more migrations and/or renbranding occurs the links to links to links could create a real mess. Being a technical issue I'm sure this could be solved.
The societal issue is the fact that many view microblogging as transient. Here today but in a month? Who cares what was said or linked to is old news or just irrelevant. I suspect that not too many people would cry out if things like pump.io only had a 6 month memory. Things move fast on social media, stuff stale dates quickly. This view of things may create a lack of will or interest to solve the linking problem.
Quite honestly I wouldn't have cared if @Evan had lost my entire identica history in the move to pump.io. for me and I suspect many others this is water cooler talk, and thus not really worthy of archival efforts.