I certainly don't disagree that we need to be a well-run, efficient club to be somewhat financially stable (although most Championship clubs lose a ton of money, turning back to the parachute payments havers vs have nots...) and successful in the Championship, but thought I'd add some numbers and context to the discussion. The Championship's financial landscape has/will change a lot with the increase in parachute payments.
The focus should be on wage expenditure, not just transfer fees. While we're probably paying absurd wages by League 1 standards this year (the accounts up to March 2017 had our wage bill at £22.6m, so maybe we're down to £15m-ish in League 1? Hard to say...), that's still a pittance in the Championship.
For reference, the always great SwissRamble is going through PL and Championship 16-17 accounts as they come in. This chart shows the wage bills for 16-17 that have been reported so far: https://twitter.com/SwissRamble/status/977106429168545792 After some inflation this year, the typical 'competitive' wage bill (with exceptions that have already been pointed out!) is probably ~£35m (about the same as our wage bill the first couple years after relegation), but then the likes of Aston Villa, Norwich, and Newcastle can handle wage bills of £50-60m+.
(You can also see Cardiff isn't necessarily the best example of a 'low-spending' club when in 16-17 they had a wage bill of £29m to go along with £29m in revenue and incurred a loss of £21m. I suspect their finances aren't much better for 17-18 as they've had a net transfer spend of ~£10m, plus/minus loan fees)
On a related note, the difference in revenues is getting staggering too: https://twitter.com/SwissRamble/status/977106343139119104 We may hit ~£25m revenue back in the Championship, but parachute payment clubs now have revenues of £75m+.
It's obviously not impossible for a lower-spending team to have some success in the Championship, and posters have already highlighted the right role models for us (Preston, Bristol City, maybe Millwall this year), and revenues of course don't decide everything with notable examples, but I suspect the Championship may slowly drift to have vs have nots with the increasing dominance of parachute payment revenues (think of it like Champions League revenue...) It won't be as '"easy" as when we were last in it. I think our parachute payments were something like £10-12m/year when we went down, versus £41m/year today.
I think Venky's have 'proven' they're willing to keep up a £15m+ wage bill in League 1, and probably incur £10m+ losses doing so, but are they willing to pay a £30m+ wage bill in the Championship and possibly incur £20m+ losses again (plus FFP considerations...)? Hard to say. (Hence I agree with the posters in this thread that we need to see the successful low-spending clubs like Preston or Bristol City as role models)