Statistics from Google etc. suggest that between 45-55% of cars on the motorway exceed 70mph on a regular basis anyway.
However, statistics also suggest that going between 60-65mph is by far the safest and, in many cases, it's actually more likely to improve overall journey times.
If someone is doing 70mph+, they're more likely to brake harshly, which like a set of dominoes causes many to slow. These drops speed down significantly.
A good example of this is the M56/M53 junction going Eastbound to Jct 14, which is regularly down to a crawl because cars drop their speed sharply when joining.
To be fair, I'm happy doing 80mph or above on the motorway, but far too many aren't great at.