Welcome Dave, I like science too. But I consider science to be about explaining stuff and replicability and falsifiability as opposed to obeying the authorities or risk being labelled a holocaust denier or flat earther or faked moon landing believer etc.
That 40 per cent figure is meaningless. Nobody knows how many people had diphtheria in 1900 or 1800 or today so it is impossible to ascribe a mortality figure to it.
In 1900 for example, for every sore throat that saw a doctor there might have been 10 or 50 or 1000 that never bothered and just got better in a matter of days without drugs or even seeing a physician. And even if they did see the doctor the chance said doctor would bother notifying the authorities was minimal. It is probably reasonable to think that individual doctors - assuming they were the sort to ever bother to notify of a condition - would only do so if it was serious (or perhaps in the middle of a supposed epidemic).
So that mortality figure of 40 per cent is nonsense. It is way lower, it could be 0.1 per cent for all we know, or lower.
The same holds true for mortality rates of all conditions which are often temporary - such as smallpox or pertussis or measles. Without incidence data - which we simply don't have - we have no way of knowing the likelihood of dying from the disease or how many people had it and how many people have it.
We do, however, know that according to the government mortality data that deaths ascribed to these diseases had fallen massively (in some cases by over 99 per cent) before their respective vaccines came along. In addition, diseases which saw no widely used vaccine (such as scarlet fever) also saw their mortality rates plummet in the same fashion. Clearly whatever vaccines did, it certainly wasn't significant in terms of saving lives.
Some of the defenders of vaccines here have claimed that the lower mortality rates before vaccines (which they don't dispute) are due to the introduction of antibiotics. There are several problems with this - such as timing of their regular usage and the fact that deaths due to measles (which is supposedly a viral condition and can't be treated with antibiotics) had fallen by more than anything else, but even if it held water it would still show that vaccines are not what saved us.
So we have no useful data for this.