This theory is pretty much vital to Historicism, that model largely can't work without it.
The cited Biblical precedent comes from Numbers 14:34 and Ezekiel 4:6. In both cases a literal period of days did happen. Numbers connected the 40 days of spying to the 40 years in the wilderness. And Ezekiel is told to do something for a period of days to represent a period of years.
This is not consistent with how the theory gets applied to the numbers in Daniel and Revelation. Neither verse justifies saying when God predicts a period of days it really means years.
Even IF I conceded that flawed argument, that doesn't change the verses that refer to 42 months or a "time times and half a time". You don't get to just say "a month is 30 days" because it actually isn't universally.
In both Hebrew and Greek the words used for month were also forms of their words for Moon. While I believe the 42 months and 1260 days are referring to roughly the same periods, I don't believe what's specifically said of the months (one of which is the time period of the Beast's reign) has to be calculated to the day, it just means 42 New Moons will happen. The Bible would not have used this terminology if it didn't want us keeping the Moon in mind for deciphering it.
What I'm saying is, unless you can find a way to make 30 years or about that also relevant to The Moon, the Hisotricist position on 42 months simply can't work.
Of the three ways this time frame is described in Revelation/Daniel, I think the "time times and half a time" is the easiest to interpret differently. But Daniel 4 uses a similar concept where if the "seven times" aren't years they are probably shorter periods rather then longer ones.
And even if I conceded all that. It doesn't change that the clear chronology of Revelation clearly does not allow the 1260 days of Revelation 11 to be the same as the 1260 days of Revelation 12, the latter can't begin till the former ends, and that transition point is where The Rapture happens.
But even following all of that flawed logic. Every model I've seen (which are all The Papacy is the Antichrist models) has the mortal wounding of the Beast ending the 42 months (usually with Napoleon's conquest of the Papal States). While I feel the logical reading of Revelation 13 is that 42 months is the time the Beast is allowed to continue following the Wound being healed. As well as that the wounding was specifically to one of the Heads.
This Historicist argument tends to be the only time when the fraudulent Donation of Constantine is actually treated as real by Anti-Catholics, as it is sometimes used to determine the start date.
Interestingly enough though, the Day-Year theory doesn't remove Three and a Half years from Bible Prophecy altogether, because Revelation 11 gives us Three and a Half days from the Deaths of the Two Witnesses till their Resurrection and Rapture.
Has any Historicist ever addressed the Five Months affiliated with the 5th Trumpet in Revelation 9? I believe their purpose is partly to echo the timeline of The Flood narrative.
I can't entirely condemn their desire to see the 6th head as not actually contemporary with when John wrote Revelation since I've argued for that myself in the past though it's not my current main view on the Seven Heads. But I've never seen a strong Papal View argument for why this was expressed to the John from that viewpoint. Mostly it seems to be expressed now days with Pope John Paul II as the 6th head, the contemporary Pope while many modern Day-Year theorists defined their views.
I feel if we accept that 6th Head as not actually when Revelation was written it must be either a time already in the Past when John wrote, something contemporary readers could have seen as significant. Or the 6th is during the Eschatological Week, perhaps reigning as it starts. As far as an already past modern historical time goes, only the founding of Israel in 1947-1949 is a remotely viable option.
The Papal fixation is dependent on the "Temple in II Thessalonians 2 is really The Church" error.
It's interesting how Historicism seems to have the least variety in it's Antichrist views. Even Preterism occasionally has options besides being a Roman Emperor.
I could hypothetically devise a Non-Papal views that can be just about as consistent with all the flawed arguments made above. It focuses on the Eastern Roman Empire rather then Western, and thus on Eastern Orthodox Christianity as the Harlot.
It begins with arguing that the Split of the Empire really happens in 193 AD in April when Pescennius Niger was proclaimed Emperor in Syria. Then the 1260 days ends with the fall of Constantinople in 1453 AD. Then, since Modern Greece views itself as a successor to that state, it's Seven Kings were the Seven Heads, and in that case the 6th was reigning in 1948. I considered that idea from my purely Futurist perspective here.
But again, that view would have mostly the same problems.
Historicists tend to share the bias Futurists have for wanting to believe the Millennium will start within our lifetime. And since they tend to feel the start of these periods needs to be after Revelation was written. It'll be awhile before any are willing to suggest there are two 1260 years periods, with the second starting about where the first will end. To me the plain reading of Revelation does not allow the two periods to be happening at the same time.
Historicism is not as inherently objectionable to me as Full Preterism, or Amillenialism. Since it does not require denying a literal Bodily Resurrection of believers.
And I am kind of rejecting the Individual Antichrist as it's usually defined. But the Abomination of Desolation I will always see as a specific event three and a half or seven years before the Millennium starts.
If you want to convince me of an Historicist model, you need to find a way to make it work without the Day-Year Theory.