Full suspension bikes have their place but too often, people buy them, who don't need them. Amongst the
"Seated pedestrian" types, who will buy a full suspension bike for £199, they assume it will be comfortable (They overlook the 12 tons it weighs). There are also the boy racer types, who might give a couple of thousands or more, in order to impress their cycling buddies. In most cases, a hardtail (Suspension forks,
esp lockable forks and rigid back) end is a good compromise for general cross country riding.
My commute to work is a mix of unsurfaced, in places rough, roads and tarmac, so I use a bike, with off road capabilities for that. However to cut down on the maintenance, it has rigid forks and an eleven speed hub gear. It runs 2.25 inch tyres and these provide a degree of cushioning, plus by standing off the saddle, my legs and arms provide the rest on the rougher dissents. Yes I can go faster, on such dissents, on a full suspension bike but you lose out on the climbs.
For road cycling, a road bike (What the layman calls a "Racer") is best, though with the state of the roads, in some places, maybe a 25mm tyre would be more likely to resist pinch flats, than a 20mm tyre (A customer recently pinch flatted a low profile car tyre, on a pot hole). A good few years ago, I did a ride, on a mountain bike, with locked out suspension forks and low drag off road tyres. I was interested to see how they compared with my road bike. I averaged just under 19mph, was a shade quicker on a fairly heavy road bike, with lights, tools, full mudguards, pannier rack and 25mm tyres (An "Audacs" bike; ideal for road riding in "British" weather). Then on another road bike (On which I have put 25 miles into one hour), I did the same ride, in similar conditions, at over 23 mph average.
As to longevity, aside from the rust issue (That Waxoyl can address), steel will outlast aluminium and carbon fibre. However, due to fashion, most serious cyclists are not interested in keeping a frame for forty years (Those days are gone). Thirty years ago the back pages of Cycling weekly was full of enamellers, who tended to be busy during the Winter months, when cyclists would get their steel frame re enamelled (Sometimes they wouldn't collect and pay for them until March

). There are not many left now, though
http://www.argoscycles.com/ still do it (And they pre date the catalogue shop).