I've changed my opinion on weapon retirement now that we're getting into the mid 32nd century. As Clantech becomes more prolific in the inner sphere (and now that mixed tech designs are Standard tech), I'd like to see the rise of merged tech bases. Take the best from both and let the rest fade out of use - for example, the clan medium pulse would become the new standard while the IS med pulse and x-pulse would fall out of production entirely over time. This would actually go a long way in reducing tech bloat.
It's already starting to happen anyway
Number of canon variants with the following IS weapons since 3075 (of over 750 IS/Mixed Tech Mechs, Combat Vehicles, Support Vehicles, and Aerospace Fighters) Source: MegaMekLab's cache (i always delete the 'unofficial' .zip so those aren't included)
AC/2 - 5
AC/5 - 2 (one is a combat-modded industrialmech)
AC/10 - 3
AC/20 - 2 (none after 3078)
PPC - 13 (vs 75 ERPPCs, ~80 HPPCs, 68 SNPPC)
Large Laser - 8 (vs 70 ER Large and 27 LPL)
So since the height of the Jihad, the release of new tech is clearly reflected in design choices - IntroTech is largely being phased out (SL, ML, and MGs are still hanging on, but barely). The next logical step is to begin replacing the 3030s-3060s tech upgrades with their clan equivalents. It seems that this might alleviate one of the chief complaints I hear about playing in later eras - too much gear to keep track of.