You're missing probably the most important question here around whether another carrier will work for you.
If you live in East Manhattan and both EWR and JFK are roughly equally convenient, I'd say sure.
If you live in the middle of nowhere and have to connect to get anywheres anyway, I'd say sure (with an asterisk on United's reliability - which, while it has gotten MUCH better the past 4 years, still isn't at Delta's level - but as a Platinum you already know that).
If you live in MSP or ATL? Good luck with that, unless almost all of your travel is international to secondary cities and requires at least one stop anyways.
If the above is true.. I think it is a no-brainer to try to maintain status on both United and Delta. More upgrade certificates, more award options, and so on.
I would actually argue whether it would make sense for you to concentrate your flying on United to get Global Services there as United GS is actually useful (as opposed to DL360 which is kind of useless).