I am not sure if I have understood the problem properly, as I cannot see how
an IF statement would help you no matter how many IFs it had.
As far as I can see, what you need is simply a formula in each blank cell
that equals the cell above it. So, in B3 you would need =B2. If you were
then to copy this cell and paste it into B4:B8, the formula would adjust so
that, for example, in B8 you would have =B7. But you need to paste this
formula into all the blank cells in the range of 7000 cells in column B. To
achieve this all in one go (almost):
Copy B3.
Select column B.
Use Edit > Go To > Special > Blanks > OK
Paste.
I said "almost". This process will paste your formula into all the
intervening blank cells. You will have to do the last section (below the
last entry) manually, as Excel will not know how far you want the last entry
copied. (The alternative would have been to put something temporarily in the
cell immediately below where you want the last entry copied before starting
the procedure described above.)
If I have misunderstood your requirement, I apologise. Don't despair; have
another go at explaining.