We just got off the 12 night Southern Caribbean sailing on Grandeur. I read the OP's review just before leaving and was rather disheartened. Thankfully, we did not have the same experience.
We prefer the same ships and are well aware that most of them need to be updated. You can see wear and tear on furniture both in the cabin and around the ship. This is to be expected when these ships are 20+ years old and I absolutely agree this needs to be addressed by Royal. However, I did not think that the Grandeur was "filthy and ill maintained". It seems that all day, every day, there were crew members cleaning in the public areas but indoors and out.
We ate in the MDR most nights and found it to be quite good. The food was hot when delivered and we enjoyed the variation the menu. On the nights we ate in the WJ, we also had no complaints. I did not seem to us that they served the same food every day.
We did not go into the solarium, fitness room or sit on the pool deck so I cannot address those areas.
My problem with the linens is not that they are threadbare but that they are SO ROUGH! I have a problem with this on every ship.
Since we cruise with Royal a lot, we had seen several of the entertainers and production shows before. Nothing outstanding but fine, especially for first time cruisers.
We also found the app to have incorrect information on many occasions.
One thing I absolutely agree on is the WIFI. It was horrible. We had no WIFI service in our room and had to go the Schooner Bar to sign on.
Even though it is old and could use some TLC, we would not hesitate to go on the Grandeur again.