Admittedly, this is a suggestion that only a large, committed organization can implement, but it is sane:

Drag email worthy of retention into an Enterprise Content Management (ECM) system.

You can find ECMs nowadays that are well integrated with your email client (and may even allow users to ignore that they are moving information out of Exchange). You can think of Exchange as merely a staging area for all incoming information; anything with long-term business value should be expected to end up in the ECM where it can be shared, portalized, linked to wikis or other knowledge management applications, filed by project or business area with related information in other content types, searched six ways to Sunday (Exchange is woeful at this), etc., etc.

If you're able to do this, then you shouldn't miss the other 95% (I'm pulling that figure out of the air), the dreck that's left behind in Exchange, when it's aged out and shredded.

If you're not able to do this, and you're trying to adapt your email repository as a long-term knowledge archive, at best you'll have an individual repository for yourself, not your team or your organization, that can't be easily shared or integrated with other related knowledge.

That's why you shouldn't mind the retention limits established in Exchange. As to why they should be established at all, consider that the organization has no business interest in maintaining a gigantic archive that is 95% worthless, poorly organized, poorly indexed, unsharable crap and that, if a legal discovery request ever does hit, will be an unbelievable burden to sift through. Plus, the retention deadline gives people an added incentive to move the valuable stuff into the valuable place.