...coupled with the automatic revocation of the older license keys for v5, so that the v5 product can no longer be installed with that old now-dead original v5 license key...
Well, that just bites my ample asset. I always thought the licenses for a particular version were free for life (you were just locked into that version).
I said that if you take advantage of the "free upgrade" they are offering for v5 -> v6 for purchases made in the past 3 months, then they're giving you v6 "for free". They are replacing your now obsolete v5 with a brand new v6, for free. Why would you care anymore about v5, since you just opted-in to their offer for a "free upgrade"?
If you'd rather stay with v5 and its old (but still valid) license key, then stay with it and don't take advantage of their free upgrade offer. The voiding of your v5 license key (so that you can no longer install v5 with this key) only happens if you tell them you want to upgrade to v6 in which case they send you a perfectly usable v6 key to use going forward with v6 software. V5 is dead to you.