As a Porsche nut I would dearly love to see them in F1. However I don't see that F1 is a good marketing tool to sell cars these days. I don't think people buy Ferraris because of how they do in F1. Less so the people that buy Renaults and Toyotas. Modern F1 teams are businesses in there own right that exist because of the money that sponsors pay them. The VW bean counters will have already worked out that spending money on F1 is unlikely to produce any return in sales.

On the other hand I have to hand it to Porsche to run a high profile series like the Supercup at GPs and get teams to buy the cars of them and advertise their brand!