Well, I imagine a fair few of the teams would be barely able to survive and most of the additional teams would be right at the back of the field. In 1989 we had lots of no-hopers in the form of current Marussia/Caterham. Just the unreliability rates were higher, so sometimes they even got points. In 1990 we had that famous Life as well.
I don't think the IndyCar elite would be in F1. In 1989 the CART stars like Andrettis, Fittipaldi, Rahal, etc, were still in America. So I expect Power, RHR, Montoya, Castroneves, etc to make a career in IndyCar rather than "make up the numbers" at the back of F1 grid.
Perhaps some of Alguersuari, Buemi, Kovalainen, Petrov, etc would indeed be F1 like currently Kobayashi - be the experienced lead driver of a backmarker team.
Perhaps some from GP2 or other feeder series, who have hanged on for long there already, would have made it into F1, like Cecotto jr, Palmer, Valsecchi, Leimer, etc. But they would be gone after a season or two again, just like D'Ambrosio, di Grassi, and others were. Although were there more seats, some of them could even have a sort of a career there. For example like Piercarlo Ghinzani, Nicola Larini or Gabriele Tarquini had in late 1980s, early 1990s. Drove year after year in rubbish cars in F1.
And some guys from current DTM, who were previously in some junior formula series, but couldn't continue pursuing F1 dream at the time.
So yeah, basically we'd have more of the di Grassis, d'Ambrosios, Pics, Sennas, Petrovs, van der Gardes in F1, with some of the more experienced/older drivers in the mix as well for good measure. Remember, in 1990 38-year-old Bruno Giacomelli made a comeback in Life to have some fun.