I don't disagree with your sentiment, but the MBP you reference with the LG 5K would run you about $3900 ($2600 MBP + $1300 LG) versus $2500 for the iMac. If you do not need a laptop, then the iMac is the far better value here. Faster in every metric and costs less.
What I meant is for the price of just the high end mbp and LG 5k you could buy both an iMac and MBP.
You wouldnt need the 5k monitor if you had the 27" iMac. 15" for power on the go and iMac for desktop use. With the specs above you could buy both mid range for one high end macbook pro and 5K display.
2.6ghz 1TB 32gb vega 20 £3734, LG 5k £1179, applecare £399 = £5312
2.2 512 16gb 555x =£2529
5K iMac 3.8 8gbs 512 580 = £2429
Total of £4958 Saving of £354
Still leaves you with £350 to spend so you could upgrade the iMac ram to 32gbs for £260 giving you 40 in total or buy the i7 CPU upgrade on the iMac. Or get a ram upgrade on the MBP or even buy the mid tier 2.6 option.
Just shows what poor value the top end macbook pro with a 5k display is.
Problem is this equation wont exist with the 2019 iMac because pricing will be in line with the 2018 MBP.
If you were so inclined you could get both on refurb pretty easily
The above MBP is £2229 on refurb
You can get the 5k on refurb with i7 512 8gb 580 for £2539
Total is £4768 £544 saving on the 2.6 1tb 32gb vega 20, Applecare and 5k monitor.
You arguably get a laptop which is within 10% and a desktop which is faster and looks far nicer than that awful 5k LG display.
Crazy when you do the numbers.