There is lots of room for improvement left at the lower end of the phone/tablet market but phone and tablet manufacturers are trying to stretch their high-margin products for as long as possible so the lower-end is not going to see significantly improved products any time soon.
Take Samsung's Galaxy Tab A 7" launched a month ago: it is a slightly updated version of the Galaxy Tab 4 7" from 2014. Same 8GB eMMC, same 1.5GB of RAM, same 800p display, sidegraded (maybe) from a 1.2GHz Mediatek 1088 to some unspecified 1.3GHz 32bits SoC, updated cameras, lost dual-N support along the way and retails for $20-30 more.
There is less than $50 worth of parts, manufacturing and development costs separating $150 tablets and phones from $500+ models and most major manufacturers are trying very hard to keep the brakes on against the race to the bottom. Asus' $200 Zenfone 2 551KL is one example of how scary the $200 price point could become for premium device manufacturers.