I agree the existence of this management behaviour! However (IMO) the very holding of this belief is feeble in the extreme ... and is a clear marker for a manager who knows they couldn't organise a beer in a brewery.

Microsoft and most other major vendors don't care whether you have a mixed environment or not ... mostly <cynic> they just don't want to fix your "problems" unless obligated too (under a contractual obligation) or in their opinion (not yours) there might be brand damage if it is left un-addressed </cynic>. This is particularly the case now the security thing is sucking up most the discretionary oxygen vendors have to address customer use case problems.

I once worked in a shop who had a platinum support contract, were poster pin-up spruikers for the vendor and was suffering a nasty operational risk/loss in a flagship product offering ... that was comprehensively analysed and documented and presented to the vendor for a patch ... and still they skirted around accepting any responsibility. Net outcome was apologies to impacted customers and various band-aid patches to code to keep it within manageable levels. Very unbecoming in a professional sense!