This is clearly off-topic gone wild, but let me add my 2¢ anyways. 
That 16MB flash thing is not only economical, but also technical. If you take a close look on the different RotuerBOARDs you’ll notice that all those 16MB flash devices use SPI Flash chips, whereas devices with a larger amount of flash usually use NAND flash chips. And that’s significant. For IPQ4018 SPI flash is the only available option, whereas IPQ4019 also supports SD/eMMC and ONFI interfaces. But as with a lot of other microcontrollers, some pins are usually shared between different interface, so you have to chose what to use. Have a look, for instance, at hAP ac3 and hAP ac3 LTE6 kit- the former has no mPCIe slots and 128MB NAND flash, whereas the latter has mPCIe slot (with LTE modem preinstalled) but only 16MB SPI flash. Are those related? I don’t know for sure, but I guess they likely are.
For SPI flash, 16MB (128Mbit) is the maximum you can access using a “traditional” 3-byte addressing. I know (I googled) there are IPQ4018 devices on the market with 32MB SPI flash chips. What stops Mikrotik from doing the same? I don’t know. There may be some hardware compromises to consider, or it may require architectural changes, or… I am just inclined to believe that they have a good reasons for that.