A report from up the back in economy. I do think the cabin manager has a very strong influence on the service during a flight. On recent flight SIN-FRA (day time flight), the scene was set at the start of the flight for us. As Solitaire PPS status, we usually get a welcome on board at the start of the flight, along with a bottle of water. I'm not a DYKWIA person, so whether I get the welcome or not is neither here nor there. However - it does set the scene for the flight and does show how well the crew are being managed. Sometimes you get welcomed by a flight attendant looking after your area, sometimes it will be the cabin supervisor, sometimes the overall cabin manager and often it will be 2 of these or all 3. IME, there's a strong correlation, the more welcomes the better the service (for everyone) in the economy cabin for the whole flight.
On this flight - no welcome at all (nor drink served with meal first which is also a usual "privilege"). Throughout the flight the cabin crew seemed disorganised, not a lot of cabin patrolling or water runs (particularly on daytime flights SQ usually proactively go through the flight with trays of water and juice - only saw 2 of these in 12+ hours). At one point in the toilets I noted there was no toilet paper and they were very un-SQ-like in terms of cleanliness. My working theory is poor cabin management = poor service.