The downside of BIP68 as written is users of by-height locktimes have 14
bits unused in nSequence, but by-time locktimes have just 5 bits unused.
This presents an awkward situation if we add new meanings to nSequence
if we ever need more than 5 bits. Yet as shown above, the extra
granularity doesn't have a practical benefit.


Recommendation: Change BIP68 to make by-time locks have the same number
of bits as by-height locks, and multiply the by-time lock field by the
block interval.