This ^^^^^
We need to stop thinking that canal closures, even on busy routes, are unthinkable - if money is short, it will happen.
I'd say the most vulnerable are big embankments and tunnels (aqueducts less so but they often have big embankments on their approaches) - followed not far behind by big cuttings. A large failure on one of these will close the canal indefinitely and almost without warning.
Thinking about it I would add Marple locks - failure rate is one every few years at a couple of million quid every time.
I have reports on my shelves from the 50s and 60s that look at permanent closure of some through routes including Harecastle Tunnel
Even though I used lock gates as an example, they are not actually the weak spot they seem, the limiting factor at the moment is the capacity to build them, certainly Bradley are working flat out, and I suspect Stanley Ferry are too, but capacity simply means waiting for new gates not them never being built.
Another big problem is that reliability will be much reduced, and that is critical for the hire boat industry and canal side businesses dependent upon the leisure side of boating. Before maintenance was such an issue, we used to reckon as a rule of thumb that the leisure trade needed the canal to withstand the 1 in 10 year drought*, that is. so long as the failure rate was less than a 10% probability it would cope, if failure rate, any failure, starts to exceed 1 in 10, that is a 10% chance that holidays will be disrupted or cancelled, it will be very bad news. The public may be even more fickle now as there are more alternatives.
*This gave concern on the Rochdale where the water supply was only up to the 1 in 3 year drought. In practice usage has not been as forecast so this hasn't been the issue we thought it would be, and our understanding of water management and supply has improved so we were probably being unduly pessimistic