Weight difference is minimal, size difference is minimal. Yes, there is a difference, but I don't notice it really. And I'm someone who actually lugs the laptop around, on buses and trains and while riding a bike. Most people don't even really carry their laptops, maybe from car to office and that's it.
As a person who actually carries their laptop around, I appreciate the swappable battery. Train rides are much less anxious, no need to find a seat with a plug nearby, because I know I have an extra battery.
The T14 is definitely less portable. There's less ports as well, which is quite annoying. The only downside of the T470 is the screen - I like to work outside and a higher brightness screen is crucial. If I could have the T470 with a better screen and AMD CPU, I wouldn't bother buying a new Thinkpad.
I don't know why they call ultraportables ultraportables. Are they marketed for people who sometimes lift their laptop off the table and who are bothered by the "extra weight"?
The newest T14 is more expensive than my Thinkpad from the same series was - as extra thin, hard to repair notebooks usually are. It is a bit slimmer, but in the display part, not the base with the battery. It is a bit lighter, I'm guessing because of a plastic case? I'm not sure, but the case material is noticeably absent from marketing material and the datasheet, while in the case of mine both mentioned the magnesium case and internal metal cage.
Of course that's just a surface-level comparison. But after my really bad time with the Surface Pro I am absolutely not going to buy a laptop with integrated battery as long as I can avoid it. It's just not worth the pain if something goes wrong.
Unfortunately it's becoming more and more difficult to find removable battery equipped laptops, what current model lines do have that still?
Personally, having backup batteries is a requirement for the places I work.
Laptop being thinner is a trade off you're willing to make, I am not.
Charging a battery with another battery is extremely inefficient.
Lugging around laptop batteries is exactly the same thing as lugging around power banks, except laptop batteries don't require a dongle and a cable to tether you to a table.
Laptop batteries mean you laptop can continue to in fact work from your lap.
A powerbank for the same price will probably have a higher capacity, and be able to charge any USB-C device.
All this is not a reason for integrated batteries, but Powerbanks using a universal interface are clearly more practical.
Battery replacement is a once-in-a-few-years operation and it's OK for it to take 5 minutes.