The grave cannot praise thee (cormpare the comment on Isaiah 38:11). It is avoiding the plain force of these passages to say that Hezekiah only means that those who go to Hades in a state of condemnation cannot be expected there to praise God (Kay).
He speaks broadly and generally of all: "The living, the living, shall praise thee; Sheol cannot praise thee; Death cannot celebrate thee." Manifestly, though he believes in a future state, it is one in which there is either no energy at all, or at any rate no devotional energy.
He may think, with Isaiah. that "the righteous man," when he is "taken away," will "enter into peace" (Isaiah 57:1, Isaiah 57:2); but absolute "peace" precludes energy (see Arist; 'Eth. Nit.,' 1. 10.
§ 2). Hezekiah shrinks from losing all his activities, including his sense of personal communion with God. He does not, perhaps, "look on the condition of the faithful departed as one of comfortless gloom;" but he views it as one of deprivation, and is unwilling to enter into it.
It was by the coming of Christ and the preaching of his gospel that "life and immortality" were first truly "brought to light" (2 Timothy 1:10).