Disagree personally. Any club with limited resources that wants to become successful needs a well developed longer term strategy that stays in place beyond the tenure of a manager, that dictates how the whole club is run - from recruitment and style of play through to how the reserves/youth set up is run and the infrastructure decisions like the pitches, or even the interplay with the medical teams. That can't fall to the manager as their decision making is weighted towards the short term (fair enough given how they're treated) and can't be the owners as it should be a full time job. Can't be a CEO as it should be someone that understands football first rather than numbers first. Try telling Brighton / Brentford et al that we don't need a DoF and they'll laugh you out the room. It's why those clubs - and many more like them - can see a manager or key player go and continue progressing without the house of cards falling down.
As for SLF's record - I'm of the opinion that we can't ever really know how well he has or hasn't done. Clearly a lot of our managers have signed 'their players' like Wallace/Kelly under Warbs and JCS/Balogan under The Snake, rather than well scouted and data-driven signings. That shouldn't be happening if we are recruiting properly. However, how much did SLF get undermined by the owners with managers going over his head? Or how much was him being weak and giving the managers too much sway over recruitment? We just can't know without inside information. My guess, and it is just a guess, is that SLF did a decent job given the challenging owners that we have (budgets and league finishes progressed reasonably until last year) but wasn't quite strong enough as a personality to hold managers back at times.
A new DoF is now a critical appointment in my opinion, and naturally I have absolutely zero confidence the club will get it right.