May I try, once again ... perhaps with a little more detail ... doesn't matter what heavy we are considering .. the basic rules are along the following lines ....
(a) the limiting design speeds are EAS as we are interested in forces acting on the airframe .. in particular ... ripping off D-cell leading edges, aeroelastic problems and such like.... which are the sorts of nasty things which might happen if one is unduly careless or wilfully naughty.
(b) Vmo/Mmo, amongst other things, are related to the design speeds .. hence a basic interest in EAS values at lower levels and mach number at the higher levels ...
(c) the ASI is a mickey mouse, albeit fancy, differential pressure gauge, calibrated in IAS on the basis of certain assumptions as to what the air properties are .. and we all know that the air properties vary in the real world .. therefore the ASI has lots of real world errors which we allow for with the usual corrections
(d) the ASI reads EAS at sea level standard only
(e) as the aircraft goes UP, the ASI reading goes UP for a constant EAS .. ie the ASI tells fibs ... the main reason that the AFM and OM speeds are in IAS or CAS rather than EAS
(f) in general, you will see the Vmo and Vs lines increase once these errors become significant .. typically above around 10,000 to 15,000 ft .. the ASI errors cause a noticeable divergence of the IAS reading from the desired EAS
(g) the specific constant IAS limit referred to in the earlier post will be due to some other reason causing Boeing to restrict the EAS, or even a design or operational compromise within the design camp not related to the basic design standard. We would need some comment from a Boeing chap or chapess to resolve the specific detail .... and, it doesn't really matter, the basics are not altered
If you are interested in looking at the current basic design rules, try FARs 25.253, 25.335, and 25.1505.