The back 1/4 of the starting 20 is usually pretty uninteresting anyway, not sure what adding 10 cars would do. In Nascar there's greater parity, but adding 10 more F1 cars likely wouldn't mean there's more good cars in the back.
Plus it costs about $30 to $50M to run a full-time Cup car. It costs about $200M to run an F1 team (two cars), with the top teams costing more. I think the dynamics are just very different between the two. Personally, I like 20 cars in F1, based on 10 teams. I think it makes it more competitive, not less.