It's hard to discuss a monitor being 100ppi@720p because those 2 variables are related via a third variable size. A 720p screen would be 100dpi at about 14" and indeed that is a common size for crappy laptops. A tiny screen with that PPI strapped to your face would look like shit. For instance a 2" square screen would be 200px x 200px.
A nice high end phones screen is like 300 PPI a 20 year old laptop screen is around 100. A nice desktop monitor is 160-240.
A calculator https://www.sven.de/dpi/
It's hard to discuss a monitor being 100ppi@720p because those 2 variables are related via a third variable size. A 720p screen would be 100dpi at about 14" and indeed that is a common size for crappy laptops. A tiny screen with that PPI strapped to your face would look like shit. For instance a 2" square screen would be 200px x 200px.
Resolution matters a lot.