If you can do that at Google's scale I'm sure they'd be happy to hear your proposal. I really doubt Google's engineer wanted to give up the precision (they'll use it for their daily job as well), I just think they couldn't justify the additional resources.
Don't make it sound like a lack of ability. They could keep giving time slices to the old code if they wanted to. And it wouldn't that that many resources, "justified" or not.
Keeping old code running within Google is surprisingly hard...
Most big services (eg. Google Docs) have entire teams of people who run them (SRE's). Without those people to keep the system going, it would probably break within a few weeks.
