This depends on where you fly…
I can see someone flying wholly within one or two countries being able to manage just with LPV. But to spend that much money, not to mention effort installing complex kit which can give trouble (capability never comes without strings attached – just look at the countless forum posts talking about autopilot problems and how almost nobody in Europe can fix them) and not get ILS, seems a waste to me.
You can still fly the ILS manually I imagine.
Simon
You can always fly an ILS manually, it is not that complicated ;-). Or you use the GPS overlay of the ILS, does work also.
Yes, there is a load of discussion on autopilot troubles, but mainly for the analogue ones and yes, that is an art not many are left to understand. For the digital autopilots you have to understand their function as well and especially the new ones derived from the Experimentals world require quite some knowledge to configure correctly, but it is not rocket science. Btw: installation is by far not that costly and work is not heavy.
Time = money in the certified world, but not so much in the experimental one. For instance there are nearly 30 pages devoted to A/P setup in the G5 installation manual.
Simon
Time = Money, especially in the certified world, agreed.
Forced to read 30 pages for an autopilot setup only at the G5 + the same(?) for the GFC500 eats my time = my money, though.
You can indeed fly an ILS manually, even with a Yaesu handheld, but try doing it in real wx, 40kt crosswind, rain, screaming passengers, partial loss of avionics, etc.
I wonder why this limitation?
You can always fly an ILS manually, it is not that complicated ;-). Or you use the GPS overlay of the ILS, does work also.
Autopilots are there to reduce your workload … especially in bad weather situations..
Not to mention that charted distance might be different between ILS/DME and GPS and a GPS overlay might not be available.
There was even a flight checker that crashed into a hill a couple of years ago after first time calibration and being
confused about the correct distance to the threshold …
I wonder why this limitation?
Very easy, it would have caused at least one to two more years until certification. Get certified on GPS is easy, do it on the GPS data stream format once for all GPS navigators. Get certified on radio navigation equipment means doing a lot more test flights and tweaking of any NAV equipment subject to the STC.
I don’t think it is anywhere near that simple. See e.g. this thread from a few years ago.
Decoding the VHF/UHF LOC/GS signals is trivial, to extract the analog deviations. After downconverting the RF, you can do it all in software with common DSP algorithms. No tweaks or user adjustment needed; the output is well defined in terms of the input signals.
Also there is no such thing as a digital autopilot 
It’s a marketing phrase, with little rooting in technical reality… well since the 1970s, anyway.
As pointed out by Sir_Percy above, there is no general way to replace ILS with GPS. Somebody would have to generate a whole database of GPS waypoints which give you the same LOC and the same GS. Nobody is going to do that – see here. So I would suggest that if a vendor is saying you can use GPS in place of ILS, that is like me being told by the UK Cirrus importer, 2002, when I asked if they can fit a DME, that a “GPS is much better than a DME” Obviously, I spent the 200k with Socata instead.
Ahhh, Good old times when a new SR22 could be had for 200K :-)
