I'm not condoning the practice of GPS let-downs HOWEVER, the current ground based instrument approach procedures are based on systems that were invented and certified decades ago. (It's not that long ago that we did DECCA approaches IMC in non-coupled dinosaur helicopters!).
Corperate twins now have duplex three/four axis coupled autopilots, multiple GPSs with terrain database (some have DGPS), TCAS, (some have weather radar and GPWS), NDBs, VORs, DMEs and more.
Whilst I agree that in principle it is feasible to design and approve IFR approaches to private landing sites, your statement above really is only half the story. Aircraft systems have improved vastly over the years, but this is not the sole criteria for an approach. I refer you to PANS-OPS and the details of design of approaches. Obstacle clearance is the primary concern when developing an instrument approach. Generally oil and gas platforms are placed with an HPZ (helicopter protected zone) or HFIS, so any traffic or new structure are known and notified, and there is a natural limit to the amount of building and obstacles that suddenly appear offshore. Not the same onshore, where people can in a relatively short space of time erect a large telephone mast, crane, fly kites, use fireworks, shoot weapons etc without too much prior notification or permission required, vastly increasing the chance of a "surprise" to the pilot on an IMC/IFR NPA letdown onshore.
Further to this, whist indeed the procedures are based on systems invented many years ago, they are still in essence the same ground-based systems we use today, especially non-precision, NDB or VOR. These require are large degree of maintenance and constant checking for certification. Again this is entirely possible for a private site, but would be costly, and certification outside a control area/zone for an approach would be potentially difficult, if the operator has no control over the area surrounding and therefore unable to guarantee the obstacle clearance values. It is possible though, and I believe, if I remember correctly from many years ago, uncontrolled, unmanned airfields with NDB approaches in Australia, but these were generally in very sparsely populated areas - in which the UK would probably fit about ten times!
Of course there are also many other requirements relating to descent gradient, fix tolerances and the effect of the surrounding terrain, to start the list, so read PANS-OPS vol 1 for starters, then if you still have the will to live give Vol 2 a go, to see what I mean, and you will see it is not as straight forward as just having a modern aircraft and a "really good" pilot!
Once we have the approach procedure nailed for our onshore private site, we have to get the missed approach sorted, and this is again fairly detail intensive, with regard to obstacle clearance and Cat A operations and performance criteria. All entirely possible, but I suspect there is good reason we do not see it in practice very often onshore.