GA pilot reported staffing issues at SQL Federal Contract Tower that seem to be getting worse and believes it to be a 'dangerous' situation. Reporter stated the Tower is closing on some afternoons.

Narrative

The Control Tower has experienced staffing issues in one form or another dating to before 2019. Even in the best of times; retaining knowledgeable Controllers was difficult and we often have a revolving door of inexperienced and/or unfamiliar Controllers. Recently; the staffing issues have deteriorated yet again (as they did in 2019) to a safety-of-flight issue. My opinion is the situation has become dangerous. The Controllers simply sound stressed and distracted. I do not remember the last time Ground and Tower frequencies had separate Controllers.SQL is located directly in the final approach of SFOs 28R/28L; we have a fair number of IFR operations; and we have a complicated VFR-to-IFR departure procedure which is confusing to transient pilots. At best; it takes several minutes to obtain a clearance; read the procedure; and obtain a readback. Having a single overworked Controller is a predictable failure. The Tower regularly limits closed-traffic operations during some or all hours of a day. This month however; there has been a new escalation as the Tower is simply closing operation some afternoons. This results in confusion and an uncontrolled field in a major metropolitan area; in critical airspace. This is a dangerous situation with a predictable outcome and a rather simple solution.