Since the Raymond Burr-led Perry Mason went off the air in 1966, at the end of the black-and-white TV era, there have been two attempts to revive the character with other actors. Neither really lasted. I haven't seen either, so take this all with a grain of salt. But I can sort of see why they haven't been successful.
First there was The New Perry Mason in the early 70s. It starred Monte Markham, who's still with us at 90. Again, I haven't seen it, so I can't definitively speak to quality. But it was probably a little too soon. The old show had been filmed and preserved on video, and was now slipping into syndicated reruns. Viewers could be forgiven for doubting they needed a new one.
The HBO version is of course much more recent, about five years old now. Matthew Rhys is certainly a good actor, although by this point he was a little long in the tooth to play a Mason who hadn't even started practicing law yet. That last part seems to have been part of the problem. Moves like changing Paul Drake's race and making both Della and Burger gay/lesbian might have gone over, or at least one change like that could have. But anyone with a previous affection for the character wants to see Mason in the courtroom defending the innocent, not rooting around generalized corruption in interbellum America.
The character does have an enduring appeal, based on decency and an admirable commitment to justice. I wouldn't necessarily say that Burr is irreplaceable. But he would be very difficult to replace. History seems to show that.