As a Constructor, Porsche was 3rd (1961) and 5th (1962).
Exactly. Not brilliant as a constructor. And therefore of questionable suitability as a brand for the VW Group to enter F1 under really.
A stronger argument could be made for the suitability of Audi, because of their historic ties with Grand Prix racing under the Auto Union brand. But even then, Audi, much like Porsche are more associated with Sportscar racing now.
But still
an entry under the Audi brand would pale in comparison to the marketing opportunities of a true, contemporary Auto Union brand
(Audi, Bentley, Bugatti, Lamborghini, Porsche, SEAT, Skoda, Suzuki, Volkswagen)
marketing gold mine right there, tailored for each race, tailored for each country.
They did win the WC powering McLaren F1 cars in 1984 and 1985 and finished 2nd in 1986 and 1987.
True, but I don't think they'd be much mileage for a VW brand entry purely as an engine supplier nowadays.
Strong privateer teams like Williams and McLaren were the real foundation of F1 for the 1980s and 1990s along with "factory privateer" teams like Ferrari and Lotus (I'd consider the current McLaren to be a "factory privateer"). They're successful and large enough to survive the manufacturers coming to play so when those manufacturers leave, there is still a solid core of teams left to keep it going.
Technically Lotus wasn't a factory team, it was a separate entity independent of the road car company. McLaren has certainly become one. They're thriving. It's sad to see what Williams has become though. I'd like to see a competitive Williams team again.
But I agree completely with what you're saying regarding factory and privateer teams though.