I don't think the real reason is that the market is just saturated.
2014 MBP 15" base model had 16GB / 256GB SSD, so I would expect that after 4 years, the base model would have at least 512GB SSD, because the prices are much lower. The higher-end 2018 model should have 32GB / 1TB SSD.
I have 2015 MBP 15", it works well, it's my main workstation, but I planned to upgrade. However the RAM and storage are the key specs for me and the base 2018 still have the same RAM and storage, so why on earth would I upgrade (and also pay +1000$ more than I payed for my MBP back in 2015)?
I also think that they won't have to wait for Intel processors to just bump the specs every 9-12 months, so when you buy a new laptop / desktop, you know you have the latest possible technology. 2014 and 2015 had the same processor (maybe even 2013, but I am not sure about that) and I was ok with that, it was Intel's problem.
iMac 4K / 5k should already have at least one fast SSD and one slower (1.5TB = 1TB slower + 512GB faster) as the base. No, they still sell almost two years old iMac for the same price with 1TB HDD 5400rpm... it's really like naming it "areYouStupidEnoughToBuyThisiMac".
Why not upgrading Macbook 12" in 2018? Add the true tone, 16gb for as the base for the higher-end model, better screen, touch id, 3rd gen keyboard... why not also doing the same with MBP without touchbar and forgot about the whole Air thing?
All Mac computers should have the same tech inside and the difference between them should be the size & power. True tone is not an expensive tech, touch id is not expensive tech, 16gb of ram is cheap, 3rd gen keyboard are just a few condoms added. It's like with that 3D touch missing iPhone XR... why? Just make 3 sizes of XS and make them cheaper. $599 the mini / $749 normal /$899 max = everybody is happy.