From the engineering standpoint, setting up the device to detonate remotely on an aircraft at cruise altitude is quite a bit harder than simply setting it on a timer. The easiest way to make a remote detonator is to hook up a mobile phone, and those generally don't work at 30000' and 400 knots. It's possible to cook up a design that involves walkie-talkies or even a dedicated circuit, but that requires a whole different level of engineering expertise.
Historically, terrorist groups have been short of engineering talent but that seems to have changed beginning with bin Laden. Today, with the "Maker culture," a device capable of measuring barometric pressure several times a second, to an accuracy of 0.02 hPa, computing the trend line, smoothing the data and applying whatever logic and/or time delay before activating an output can be built with off the shelve parts for under $20. Powered by 2 pen cell batteries, it can run for months and is the size of a deck of cards without the explosive.
With such a device it becomes possible to count aircraft pressurization cycles so one could set the device to wait for the descent prior to landing, wait 30 minutes after reaching maximum cabin altitude, or even delay 5 or 10 flight legs before triggering. The only practical limit is how long the device remains undetected in the aircraft.
Anybody counting on technical complexity to prevent a device being made by terrorists is making a bad bet.