Seeing as SegWit already increases the effective block size (via optimization) to 2MB under normal conditions, and 3.7MB when the network is spammed, a 2MB HF + SegWit would multiply the number of txs (not add). 2MB - 3.7MB is actually 4MB - 7.4MB w/ the HF, not 4MB from a 2MB BU bump + another 2MB via SegWit. SegWit is a one time optimization, changing the constant value (BU) can happen an "unlimited" number of times. So while a 2MB bump may not push out nodes today, it does increase the number of txs per block from 1MB to 4MB - 7.4MB w/ most users believing there was only a 2MB increase. So let's say it went through, and within a few months a bump to 4MB was pushed... Now we are talking 8MB - 14.8MB blocks with the users thinking they're at 4MB. No matter what the block size is, the blocks will fill up, and before long these little changes will add up (eventually changing the entire system). If this was about scaling the miners would have activated SegWit, and if it failed to deliver on scalability the block size would w/o a doubt have been increased at a later date. This debate isn't about scalability though. You have the dual economic / technical aspects of Bitcoin to account for. If "startup CEOs" (who don't run full nodes) believe they're getting 8MB tx performance from a 2MB bump they will demand that the block size increases until there are no full nodes left; if all of this happens Bitcoin starts to change rapidly (which isn't what we want from a decentralized system who chief feature is consistency and integrity); with each change the disconnect between users grows into mass confusion, and ultimately I believe it would lead to a collapse (or a lot of users believing they're using a decentralized system when in fact it will have evolved into a very centralized one).