I know Tube Lines were supposed to be close to ordering new trains to
replace the 1973 stock, but I wonder what the status is now. With Tube
Lines being absorbed by TfL, and a government freeze on any new mainline
train orders, would it be fair to assume that this replacement Tube
fleet won't be ordered for some years?

If so, what will need to be done to keep the 1973 stock in good working
order? Personally, I like the existing trains (yes, I agree with
Boltar), and wouldn't be unhappy seeing them soldier on for another ten
years, but don't know if there are any developing
mechanical/electrical/safety problems that could prevent this. And would
they need another refurbishment or heavy overhaul programme?

There's a big gap in age between this fleet and the next oldest (once
the A, C, D and 1967 stocks retire over the next few years) --
amazingly, with the demise of the short-lived 1983 stock, the
extended-life 1973 trains will be almost 20 years older than the next
oldest LU fleet on the Central Line.

There is a large number of trains from the mid 1990s, and another even
larger batch will arrive in the early Teens. LU does seem to take
delivery of trains in a very uneven way.