If you accept the argument that fish benefit from a period of zero angling pressure (lines and angler's in the water etc.) during the spawning phase then I think it's difficult to argue that you may as well abandon the close season because there are people openly ignoring the close season law so why not just let everyone fish all year round. Whilst it means that fish never experience zero angling pressure, the reality must surely be that they are at least experiencing substantially less. This would then come down to more effective baliffing. You don't need to be actually fishing to monitor and have a presence on the banks, and generally speaking you don't need to operate in some official capacity to patrol and make an intervention. Simply saying that because there are people ignoring the law and fishing in the close season then we may as well allow everyone to do it because that will help control poaching feels a bit flawed to me.
The issue of when the close season should start and end is an entirely separate one and a bit of a nightmare it seems to me. Fish spawning at different times each year because of temperature and weather and this then potentially playing out differently on each river. And that's where it's possible to accurately determine when spawning is actually happening. It can be more obvious on some rivers than others. Plus, isn't it the case that fish can spawn more than once in a season? So how will you know when they are completely finished. If we encouraged fish to smoke then that might help I suppose. In theory you could just leave it to clubs to manage in a more dynamic way with decisions taken each year based on observed spawning activity. That would require a fair bit of organisation and it's not hard to imagine how that could get horribly out of shape with inconsistent protocols being applied across the country. You could introduce broad, national guidelines I suppose but it's not inconceivable that clubs eager for new members or under pressure from existing members, choose to apply or interpret those guidelines with a little more flexibility.
Timing wise, it might generally be the case that the existing close season needs to move because our climate has evolved and so if we want to be confident that broadly, most fish would have substantially completed their spawning, we may want to shift the dates. The problem there is that I don't believe we have the imperical evidence to support a proposal for change.