The swing-wings were created as a way to cover a wide envelope of flight conditions, with the wing technologies of the late 1960s/early 1970s.
The delta was good for very high speeds, but had diminished aerodynamic efficiency at slow speeds. A long, narrow, slightly swept wing gave good lift for T/O and landing and slow-speed maneouvers, but generated shockwaves and vortices too soon and too hard as you speeded up. A shorty, stubby conventional wing (e.g. F5, F104) or delta-with-tail (A-4, MiG 21) was a common compromise.
In the 50s and early 60s, the USA and USSR could afford to have a variety of flavors of frontline plane for particular situations but many other forces needed something that could adequately perform many different roles. Dassault’s answer for what was good enough in enough different roles was the delta. Later, the US and USSR also began thinking of planes that could handle multiple flight envelopes – but they were not about to “give up” anything so their answer was the swing-wing.
An F-14 or MiG-23 on T/O and landing had an almost straight-out narrow wing (optimized for lift), but when going full-tilt boogie became a de-facto delta wing (optimized for speed) – and the MiG goes more steeply swept in that mode than the fixed delta wings of a Mirage or an F-106.
As pointed out earlier, evolution in materials and design technologies have made it possible to design wings and lifting fuselages that are efficient around a wide envelope. Engine-thrust-vectoring technology also aids tight-space maneuverability. And computer-controlled fly-by-wire systems allow the design of planes that are not inherently stable – which was another limiting factor pre-70s : the motion characteristics of the plane had to be such that a human’s sensory-motor coordination could handle it unaided. Sukhoi is even talking of putting the forward-swept-wing on a regular production fighter.
If you look at the more modern fighters, they generally have broad-cord trapezoidal wings that “merge” into a fuselage which is itself a “lifting body” – and this in itself “stealthens up” the aircraft, since a big part of radar reflectivity is joints and such.
Makers of traditional D-wing aircraft in Europe had the fortunate situation that it is not such a giant leap to refit their platforms with the new wing technology, and you get to keep the distinctive product image.