I think this syntax is incredibly unintuitive to combine with "ago" as in the example.. what do you mean "5 yesteryears ago" that is bullshit. it means "5 years ago"? THEN FUCKING SAY 5 YEARS AGO. why include the YESTER prefix when doing that. you can't say "5 yesteryears ago" it has to be "yesteryesteryesteryesteryesteryear" to compose yester and this also shows why for longer periods than like two it's inefficient and you should just say "N years ago" but WHY WOULD YOU SAY "YESTERYEARS AGO"