Food chains can and will change. If, for example, the Trent was a dirtier, more 'polluted', more coloured river x number of years ago, then the food chain, especially at the lower end will have been very much different than today. These conditions almost certainly favoured the roach and allowed them to thrive. Subsequent changes have altered whatever balance was there then and allowed other species to thrive more so now instead.
On a similar note the Mersey in the 70's was incredibly polluted, in part due to the heavy industry around Ellesmere Port and Runcorn. The advice for a time was not to eat the resident shellfish and non migratory species. My memory may be failing me a little but as an 8year old whose elder brothers went fishing, their quarry (our food) was in the main, dabs, some plaice and eels in the summer, with dabs, whiting and codling in the winter. Sure a few other species would make an appearance but that was mainly it.
Now the river is much much cleaner, every summer sees the sorts of species caught they could never have dreamed of. A run of smoothound which can be caught from the beach, bull huss and thornback rays are also landed from the prom and good sized bass appear every summer also. However 'our kid' and his mates will say despite the new diversity of species on offer, the numbers of the old favourites caught is now nowhere near what it was.
My guess it is that like with the Trent roach, there are a combination of factors responsible, including all of the above and many many more. Food chains are subtle things, change one little thing at the bottom end and all manner of changes can and will take place further on up. However as ever in nature, the demise in one species leaves a gap for another to exploit.