I don't quite know what point you're making here. The extrapolated speeds I quote are based on dialogue and events in episodes as listed on Memory Alpha.
Memory Alpha isn't perfect.
Remember when Enterprise B didn't have it's "Tractor Beam" installed when it went in to rescue Guinan and her fellow travelers who were about to get her ship ripped apart by the Nexus. The Tractor Beam was supposed to be installed by next Tuesday.
I wouldn't be surprised if they didn't have their full complement of Dilithium Crystals or Deuterium either since their initial missions of chasing the Maqui into the Badlands didn't require everything to be loaded at that time.
Remember the Aero Shuttle wasn't even installed at the time of launch.
All parts in real life on vehicles & vessels have a limited life span, nothing lasts forever, especially in the harshness of space.
Just like Dilithium Crystals eventually do get used up even with ReCrystallization going on or the Enterprise D needing a Baryon Sweep every so often, or Phaser Array Power Cells.
In real life, many parts in your Car, Plane, Home, etc all have a finite life cycle before it needs to be replaced.
Star Trek should be no different.
So if it's the production team's own technical bible it's BS but this is fine?
It's a matter of following the technical bible vs on-screen canon, in the end, the writers choose to have Tom Paris state that line and given that Tom Paris is a damn good pilot who went through proper training initially, I'm sure his #'s are fairly accurate for the Warp Factor he's stating, otherwise, why would he lie?
When did the Galaxy class cruise speed become warp 7?
https://memory-alpha.fandom.com/wiki/Galaxy_class
It states on the side bar that the "
initial average cruise velocity" was Warp 6.
The ST:TNG Technical Manual: Section 1.1, Page 1
Warp driver coils efficiency to meet or exceed 88% at speeds up to Warp 7.0.
Minimum efficiency of 52% to be maintained through Warp 9.1. Life cycle of all primary coil elements to meet or exceed 1,200,000 cochrane-hours between neutron purge refurbishment. Secondary coil elements to meet or exceed 2,000,000 cochrane-hours between neutron purge refurbishment.
The "Initial average cruise velocity" being Warp 6 and the TNG Tech manual being Warp 7 at Maximum efficiency of 88% at speeds UP TO 7.0. That means they planned to Up-Rate the Average Cruise Velocity as it get certified through testing and in practical usage to validate planned design features. You wouldn't design that kind of specs for 88% efficiency UP TO Warp 7 if you didn't want to Up Rate your Average Cruise Velocity as the engines gets fully validated with in field use.
Even in IRL flight testing, Performance envelopes gets slowly expanded as they test and gather data through IRL usage and slowly push the engines of real life Aircraft and Vessels until they feel comfortable.
Then once it's fully certified, they continue observing data as FRP (Full Rate Production) units use it as well.
I wouldn't be surprised if StarFleet follows similar rigorous test procedures and slowly opens up the envelope for certification.
It's not like there were many Galaxy Classes initially since there was only a initial plan of 12 Galaxy Classes, so certification would take time as they expand the envelope, even through live in field usage.
Why is it safe to say that about warp 8? Do you have any evidence for this beyond personal incredulity?
Because I made logical analysis on the initial statement made by Kathryn Janeway for the 75,000 ly trip home in 75 years to the Alpha Quadrant.
Warp 8 is 1024c which would take 73.2421875 years to get home assuming a straight line trip.
You have to account for random stops, refueling, etc, so rounding up to 75 makes sense.
Warp 9 is 1,516.38110700484c which would take ~49.4599 years to get home assuming a straight line trip.
So logically they aren't planning on going at a overall average trip speed of Warp 9
That's how I came to the conclusion of Warp 8.