John B wrote:

(snip)

My guess would be that it will stay open until at least 2009, because
until the Northern Ticket Hall opens it is the only alternative exit
from the tube platforms and so closing it would be viewed as poor
safety (one of the reasons for the large death toll in the 1987 fire
was that the Pentonville Road exit was barred and staff/LFB had no key
for it, so people evacuating the station had to go up the Victoria Line
escalators and through the main ticket hall).

That fact from 1987 shines a somewhat different and more sinister light
on the "smile" motifs that appear on the walls of the tunnel walkway
that lead up to the Pentonville Road exit from the main KXSP
underground complex. It's hard to believe there was such a lack of
contingency planning.

I've no answer to your main query, though I'm most interested in
hearing if anyone else has one. I don't believe any decision has been
made publicly yet, which isn't to say that accountants haven't already
identified the closure of this entrance as a potential future cost
saving measure.

One thing to bear in mind is that the Pentonville Road entrance is
manned by FCC staff, as it's the main entrance to KX Thameslink
station. Once the new Thameslink station opens at St. Pancras Midland
Road (or whatever it'll eventually be called) then FCC definitely won't
be staffing Pentonville Road anymore. Thus if it were to remain open LU
would have to staff it, which would be regarded as an added expense. I
think this might tip the balance in favour of it closing, albeit
remaining available as an emergency exit from the KXSP complex and
perhaps also for emergency alighting from Thameslink trains onto the
closed platforms of KX Thameslink.