There is no "disproportionate punishment" under GDPR in practice, unless you're doing something egregious, and even then (see Facebook). I'm very familiar with the UK regulator, they publish their enforcement actions [1]. I'm not aware of a single case of a cautionary letter, much less "disproportionate punishment", that they sent over a cookie banner on its own. Are you?
Besides, you correctly hinted at the incentive structure. Your lawyer might advise you to slap a cookie banner just because because they have zero incentive not to, they don't care about your users' experience. You might care though. Personally I consulted multiple external DPOs and lawyers, as well as primary sources, before forming my opinion.