I seriously doubt the Pentonville Rd entrance will be closed - what would be the point?
Surely a station as busy as Kings Cross St Pancras, which has possibly the largest mixture of lines what with the different underground lines, NR lines and the HS1 routes (its such a mess on the joint Tube+Rail map), would want as many entrances and exits it can get, even if just for contingencies?
The whole Euston/KX/StP area is going to get even busier when HS2 is finished (with CR2 to come too). No harm in futureproofing by *not* going to the cost of closing an existing railway asset only to need to improve passenger flows in barely a decades time.