The 2% of total inflation is debatable but I chose it because it is the average of the whole history of Polkadot since it’s inception. It comes from the fact that the Treasury right now appears healthy and that it was able to fund projects properly through all the previous market cycles. In a sense, it’s therefore the only “guess” that is backed up by something real. Adjusting it based on price predictions for the future is adding more uncertainty.
An overflow of the Treasury is prevented by having the burn mechanism, which exactly counters potentially too much balance. What I haven’t seen much in the discussion is that burning is actually beneficial for everybody in the ecosystem, because it “pays back” to each token holder by making each DOT more valuable. So, I think it’s better to have too much DOT in the Treasury than too little (especially since staker rewards seem decent in this configuration). But a bigger balance in the Treasury should not urge people to spend it for unnecessary things. I feel, however, that managing the outflow is something that has become a spotlight for the community recently and that there are many good ideas around to make the spending more meaningful.