Could we see a streak of presidents winning reelection with less votes?

[Skill and Chance]

It's hard to see this for Biden.  Would probably require a scenario with a more mainline Republican and a populist right Nader after Trump stays out.  Otherwise, if Biden gets a non-trivially lower % of the PV, he loses.   

The idea that incumbent advantage was an artifact of the near-continuous 20th century economic boom and it's gone now does seem plausible to me.

So most presidents getting a lower percentage of the vote the second time seems reasonable to me,  but the effect would be to make reelection less likely, not create a bunch of back-to-back Obama 2012/Wilson 1916 scenarios.

[Skill and Chance]

We did also just have the somewhat bizarre situation of Trump improving his (PV) percentage and losing re-election (while obviously losing EVs, and ending up with a worse PV margin overall). Has anything like that happened before?

When Grover Cleveland lost reelection in 1888 despite winning popular vote, his vote share just marginally decreased from 48.8% to 48.6%. In 1892, he was able to regain the presidency with 46.0% of the vote.

Woodrow Wilson came within a few thousand votes of losing California and with it the entire election in 1916, although he improved his popular vote share from 41.8% to 49.2%. Of course, 1912 was a three-way race.

This is why I would focus on margins more than vote shares.  Having a high or low 3rd party vote is mostly out of the major party candidates' control.

It is notable the Cleveland got a better PV margin when he lost the EC than when he won it the first time.