I think the closest sense of but I can find in the OED is 5b:
Whilst 5a applies to:
a. Negative and interrogative sentences containing a comparative
(esp. more) were formerly followed by but; they now usually take than,
or else the comparative is omitted and but retained; modern idiom
preferring sometimes one, sometimes the other.
1713 Guardian 25 Aug. 2/2 There needed no more but to advance one
1888 N.E.D. at But, Mod. There remains no more but to thank you
for your courteous attention.
5b relates to:
b. So with similar sentences containing other, otherwise, else; in
which but is still sometimes retained, esp. after else, as ‘Who else
1589 G. Puttenham Arte Eng. Poesie iii. xix. 164 What els is man
but his minde?
1611 M. Smith in Bible (King James) Transl. Pref. 1 For none
other fault but for seeking to reduce their Countrey-men to good
1689 R. Milward Selden's Table-talk 41 Pleasure is nothing else
but the intermission of pain.
1713 Guardian 25 Aug. 2/2 Had no other Fault, but that of being
1888 N.E.D. at But, Mod. It is nothing else but laziness!
Earlier examples date from year 971.
In the example you gave in the question the sense of else is implied. What (else) is life but a series of inspired follies?
So it is a use of but which has been around since before the Norman Conquest and was alive and well in Saxon England.