What is more to blame for the suburban shift towards Democrats?

[Non Swing Voter]

On the one hand the talking point is that Rio Grande is trending R because minorities are trending R.  On the other, the talking point is that the suburbs are shifting because of demographics.

Doesn't compute.  Minorities aren't trending to Republicans.  But to answer the original question, it's clearly both but moreso the former.  Losing college educated whites has been absolutely devastating for Republicans.  They went from winning this group in the suburbs by double digits to probably losing them by double digits.  The diversification of the suburbs is just gravy for Dems.  Though it depends which suburbs you are talking about.  In places like Texas and Georgia it's mostly diversification.  In places like Virginia and Pennsylvania it's mostly college educated whites.

[Non Swing Voter]

On the one hand the talking point is that Rio Grande is trending R because minorities are trending R.  On the other, the talking point is that the suburbs are shifting because of demographics.

Doesn't compute.  Minorities aren't trending to Republicans.  But to answer the original question, it's clearly both but moreso the former.  Losing college educated whites has been absolutely devastating for Republicans.  They went from winning this group in the suburbs by double digits to probably losing them by double digits.  The diversification of the suburbs is just gravy for Dems.  Though it depends which suburbs you are talking about.  In places like Texas and Georgia it's mostly diversification.  In places like Virginia and Pennsylvania it's mostly college educated whites.

While I do not think minorities are trending Republican (in most areas I think it was a dead cat bounce more than anything), though an argument could be made.  However, even going with the minorities are trending Republican argument it is not mutually exclusive with the demographic changes in the suburbs.   Minorities even if voting not quite as solid D as they were, are still voting much more D than the whites they are replacing in the suburbs.

With that said, to answer the main question it really is a combination of the two of them.  Suburbs becoming more diverse has certainly hurt the Republicans there.  The march of college educated whites away from the GOP is also a key reason.  This predates Trump and is something we would have seen even without Trump, but he has certainly accelerated that trend.

I agree with all of this generally.  I just think it's very suburb specific and in most cases where it matters electorally, it's college educated whites doing it.  The big exception, IMO, is Georgia where the suburbs are becoming a lot more diverse and basically replacing 80-20 Republican whites with 80-20 Democratic minorities.