Castle ended some time ago and I have yet to comment on it.
That the series ended was not unexpected. It had been in terminal decline since the second half of season 6.
Why was this?
There was no attempt at continuity in the series. Beckett for example did go the prom in season one but didn't in season 6. Lanie hated her mother in season 3 yet did everything to appease her in season 7. Beckett was a model at 17 in season 2 but at 19 in season 6.
There was no research done on anything.Consequently we had Beckett being offered a job on national security despite being suspended for a lengthy time of gong rogue less than a year before this. Beckett becomes a captain after being only a detective. Castle is seen by two police officers attempting to kill 3XK yet is not even arrested!
Possibly the biggest thing however was here we were supposed to have a long flowing love story yet at no time did Castle or Beckett ever share anything in life, no decisions were ever made together and they never ever discussed major mistakes in life.
That such a once great series ended ingloriously is a tragedy.