Ah but look at the lines directly preceding this:
ALBANY
Friends of my soul, you twain
Rule in this realm, and the gored state sustain.
KENT
I have a journey, sir, shortly to go;
My master calls me, I must not say no.
Kent clearly declines Albany's request that he should rule with Edgar. Kent is obviously in no disposition to take the throne. His extreme vivacity, throughout the play, has always been in defence, or in service, to Lear. With Lear dead, Kent must soon follow his master into the afterlife.
As for Albany's closing lines:
The oldest hath borne most: we that are young
Shall never see so much, nor live so long.
These are hardly lines spoken in optimism - they are a grim admittance of the reality that Albany and Edgar face. The acknowledgement that the next generation will not "live so long" is last note - the completion of the nihilistic themes that are pervasive throughout the play.

