They would observe only specific areas, and this all the time, by flying above them. Satellites in Polar Orbit can only return every 90 minutes above the same area; geostationnary satellites are very far away.

Nope. A satellite in a polar orbit with a 90 minute period won't return to the same area for quite some time. During that 90 minutes, the Earth will rotate 22.5 degrees. That means the satellite's ground track will be offset by that amount. In actuality, it'll take several orbits for the satellite to return to overfly the same point on the surface. If you're talking about an imagery satellite, the preferred orbit is a sun synchronous orbit, which is actually slightly retrograde.

Ground tracks overview

The goal should be to use the appropriate tool for the job. Several years ago, Burt Rutan built a plane called the Proteus. It's a high-altitude multi-mission aircraft that can be either manned or unmanned. It was designed to perform missions like communications relay but it could also be fitted to perform atmospheric research, imaging, and even launching small satellites (it was the predecessor to the White Knight).

It's up to buyers to determine what makes the most cost effective solution. Satellites aren't cheap, especially when you factor in the launch costs. Still, they cover a wide area and can operate for years. Operating a UAV version of Proteus for several years wouldn't be cheap, either, so while it could be a good solution for some problems, it isn't always going to be the better solution at the expense of satellites.