Wouldn't it be quite easy to transmit a gps signal from a plane once every minute, and send it to a central server that sound an alarm if the plane is more than 20% off its designated route.
Yes. If you are building new planes, and using a new satellite system.
Modern satellite systems have high bandwidth and can tolerate that. In fact, Inmarsat who supply satellite services to airlines, say they could provide the bandwidth for about $1 per hour per flight. Old satellite systems often have very low bandwidth - the legacy inmarsat system that MH370 was using had a bandwidth of about 10 kbps shared between dozens of planes. You couldn't practically use it for a position broadcast system.
However, getting aircraft approval for electronics is difficult, due to the high level of safety required. Retrofits can be difficult and may be risky if not done to OEM quality (e.g. Swissair Flight 111 crashed after a retro-fitted inflight entertainment system for VIPs caught fire).
For things like satellite comms, how do you integrate it into an existing aircraft - does it need additional power, or signal connections; does it fit the same bolt holes, etc. You can't just go adding extra wires or drilling new holes in an aircraft chassis, like you would when you retrofit a new entertainment system into a car. It all has to be carefully planned and done to spec. And don't forget that any upgrade needs to fully support the systems already in the plane (which may be customised) and which are often safety critical (e.g. emergency communication messages).
The problem is that aircraft technology moves at a glacial pace, because of the need for extreme safety validation and testing. Further aircraft are often kept in service for many years, many commercial airlines are over 30 years old, and some commercial aircraft are over 40 years old.
The other issue is who pays. Airlines won't want to pay unless they have to. The question is which regulators are going to demand it, and what flights are they going to demand it for. The FAA doesn't have jurisdiction over Asian carriers except for flights which go to the US. It's almost pointless for domestic US flights, as the aircraft couldn't go far off course, and even if it did, it would be likely to come down on land and be easily found.
The fact that this event has caused so much interest is because it is unprecedented in modern times - in other words, it is an extraordinarily rare event. The reaction to such a rare event needs to be carefully judged, otherwise the cost of preventing it risks dwarfing the cost of the risk of it recurring.