under a future dem president

[Skill and Chance]

The Obama midterm (particularly 2010) Dem losses were an exceptional case because Bush's wins were both so narrow that they still walked away holding a ton of Bush seats in the South and in rural areas with New Deal history in general that then turned into McCain +15 seats.  All of these McCain +15 Dem seats had not seen an environment with a large GOP national PV margin since 1994, and they were hopeless for Dems to hold in the modern, polarized era once that materialized.  It really was a unique situation, combined with the much stranger history of a massive number of rural seats staying in Dem hands straight through the Nixon and Reagan landslides. 

I suppose something similar could happen down the line in e.g. Texas in the next Republican midterm if a Dem narrowly wins in 2020, but there are fewer holdover seats in general today. 

[Skill and Chance]

Most of this can be explained by 1994 being basically the only Republican wave at the downballot level since 1946.  A bunch of these seats hadn't voted Democratic for president since Carter.  They were massively overextended and 2010 was just when it finally broke down.  As others have illustrated, the 2010 pickups had little to do with map drawing.  Most of those gains came in 2014. 

As of now I expect Trump to win reelection, but if the next Dem midterm is 2022, they could actually have a net gain of state legislative seats from getting to unwind a bunch of GOP gerrymanders that year and basically not having any Trump 2020 seats left to lose.  Would be similar to the 2018 Senate races.