I have to agree completely with D-Type about this story. Porsche made the decision in 1962 to concentrate on endurance racing as the best means of showcasing their cars. They have enjoyed phenomenal success so this has to be seen as the right decision. On the other hand success in Porsche formula cars has been elusive.
The CART all Porsche 1987 car was not competitive and was quickly replaced by the March-Porsche 88P which was. The following year March-Porsches had a win with Teo Fabi, plus a number of podiums, poles and FLs. However, Porsche abandoned this promising effort after 1990. To do F1 would take much longer than 3 years to get decent results, maybe like Toyota, Honda and Ford, wins would never come. If this were to be the case, what would Porsche have to gain for $ millions invested? Very little, with everything to lose, which is why they will not be entering F1 anytime soon IMO.