I can't say food quality has declined over the years because I have not cruised that long.
However, of the lines I have sailed, the only line with an acceptable MDR food quality IMHO was Azamara. I recently sailed Celebrity and the quality of the fish,especially, was very poor.
I agree that the speciality restaurants have probably led to a decline in MDR food standards. So I know I am going to get flamed - I would close all the MDRS and only offer alternative venues - yeah I know it is not going to happen.
I would love to book a cruise line that based their rates on bed and breakfast only and included gratuites in their fares and yeah I know that is not going to happen either.
On a port intensive itinerary I would dine ashore.
Remember folks before you start flaming, I cruise for itinerary - (the ship is not the vacation) and my favourite meal of the day is breakfast.