What if someone’s only retirement plan is CPF and a small savings account, is that enough in today’s cost of living?

Should be enough. Singapore is notoriously low-cash, CPF-rich. Most survive like this. Just need to budget and track with discipline and should be fine.

If simple lifestyle with no travelling, it's good enough

It depends on your lifestyle & whether there's other passive inc

Depends on how much u can get monthly form CPF