Remember that Red Bull explored making their own engine with Mario Illien of Ilmor fame. Then they had Mario Illien work with Renault Sport to improve their engine.
Renault is still behind on output compared to Mercedes. Ferrari just now is equal to Mercedes in race output. It appears that Mercedes still has a special qualifying output mode.
So even major car companies have difficulty developing a competitive hybrid turbo engine package.
I think most F1 fans would agree it's best to have some common parts which would allow private companies such as Ilmor to produce a competitive F1 power unit. This would help lower the cost of racing in F1 and bring more teams.
i’m all for having a third party as engine mfg, and i’d love to see IlMor start selling engines to the privateers