DevOps isn't just a different way of doing things, it is a better way of doing things. Perhaps it will help me to make my point if I rephrase the question in a different context - the context of the industrial age, the railroad and the steam hammer:
Does a railroad company need Steam Hammers if it already lays track at an acceptable speed?
I mean, steam hammers from the business view are about speed and quality, right?
What does it mean for a construction company contracting for the railroad that is laying track "fast enough" for their customers without steam hammers? Which of these might apply?
They lay track at a sufficient level, no change is required
They use steam hammers but don't know it or call the steam hammer something else
They use something totally different and better than a steam hammer and we really should find out what it is (before it is too late)
They are awfully wrong: their speed and quality are below their knowledge and they will know it when they begin to lose bids to other contractors.
The answer is obviously probably #4. This hypothetical company may not see the problem with the next financial report - perhaps it will take a few years - but eventually it will become clear how and why competitors outperform businesses still manually "laying track" as it were. Sure, it's possible they have come up with a diesel powered hammer that is more efficient and less prone to malfunction or breakdowns, but not likely.
Thinking that they can outperform the steam hammer is going to get this company left behind. Sure, an individual group may have their steam hammer blow up and fall apart and perhaps that one group has such trouble with their steam hammer that it would have been more efficient in that one case to do things the old fashioned way, but they are the exception to the rule and the problem likely was that their steam hammer was poorly built and/or poorly maintained.
The industrial age is coming and the railroad company can either check out the competition and then try to get their own steam hammers, or get left behind and eventually succumb to their irrelevance and antiquity.
DevOps is no different. It's just another iteration of automation that companies can either embrace or ignore at their own peril much like the steam hammer.