Nothing really keeps nodes in check, i.e. they are near-free to operate.

What defines the rules of validity of bitcoin is ultimately the decisions of the people who accept bitcoin as money in exchange. This consensus is very sticky because stepping out of line with the crowd is usually an expensive mistake.

What defines which of the valid transactions actually happen, is the ordering and aggregation mechanism provided by proof of work mining.

100k extra nodes would not change either of these (in particular, the first).

The corollary is that things get interesting if a new actor comes online willing to exchange a lot more external value (e.g. goods and services) for an altered consensus ruleset of bitcoin.

Probably the subtlety there is: if it's someone saying "I'll accept Xcoin" where Xcoin is dramatically different in character to Bitcoin, then it being transacted on the same network layer is irrelevant; it's just a different currency. In some ways that's still true if you start with the same utxo set (see e.g. "Bitcoin Gold"). But it's much murkier when people debate small differences in Bitcoin's structural dynamics (blocksize war e.g.).