MIG wrote
[...]
If the end points works for fares, then why couldn't every point to
point journey for which a fare is defined also have a time limit
defined, such that Heathrow to Wimbledon Park wouldn't need to have
just a standard 2 - 6 timeout?

Speculation overload.

Indeed, exactly that may eventually be in place BUT the day of the week
is involved too.

quoting from
http://www.tfl.gov.uk/assets/downloa...s-zones1-6.pdf

The time limit varies between 70 minutes for the shortest journeys on
Monday to
Friday and four hours for some longer journeys on Sunday.
==

Paul Corfield has suggested that the extra routes added for full PAYG
on NR will require the fare charged to take into account intermediate
validators touched, so doing the same for timeout purposes seems
likely.

I think enhancing the Oyster journey history to include all touches
would be needed too.

--
Mike D