Hi Phil,
Not to shoot down your experience but some of what you say below contradicts a lot of expert opinion and experience, so your bad experience reloading the Rigby may be an exception to the rule, rather than the rule... I raise this merely to balance your criticism of the Rigby
If you're not a hand loader, the .416 Rigby as already mentioned is brutally expensive. If you are a hand loader, it's still not cheap as brass is quite expensive, though easily obtained from Hornady. I personally did not like loading the Rigby. In spite of Mike's sharp rebuke that I'm sure I'll earn, the brass design is poor in my opinion. It has a ridiculously small sharp angled shoulder which easily collapses if there's the slightest friction between bullet and brass during bullet seating.
404 brass is likely to be equally expensive and probably harder to find than the Rigby. Norma makes Rigby brass and is generally widely available.
The Rigby shoulder design is regarded as one of the
best in its class and 100 years ahead of its time (with respect to your opinion to the contrary).
I have heard of (rare) people collapsing the shoulder during resizing due to insufficient lube (this can happen to any caliber) but never during bullet seating. If it collapses
easily during seating as you say, then with respect, something is being done incorrectly.
I find the Rigby very easy to reload for, great case life (with Norma and Hornady cases) and very accurate for a big bore.
If criticism has to be levelled at the Rigby, it's hard to get away from the case being overly fat (reducing mag capacity) and it is an inefficient powder 'user'. But then again, no-one ever bought a big bore to save money!
Back on topic, I have a straight-stocked CZ 416 and I have been delighted with it! In the shoot out between the 404 and 416 Rig, the pros and cons are so marginal that the choice probably comes down to what's available at the time and whether the rifle with the sales tag on it tugs at your heart strings.