Old maps are an entirely different skillset than modern maps and not what I thought you were talking about when you said "get better" You don't HAVE to play old maps. You can continue to get better at modern maps and never touch older ones.
I'm defining old as pre 2010 by the way. 2010 and especially 2011-2012 have just a different type of style to today and a lot of them still feel very good as opposed to awkward 2008 maps.
For the most part though, it's not as serious effort as you'd like to believe. Old maps just aren't physically challenging or demanding generally - that's why all their star ratings are so low. Anyone 2000pp/3000pp/4000pp/whatever with varying degrees of physical ability in osu can learn to play them at the same rate as me because it's just reading.
And then you have shit like du hast, castle corridor, or chevron DT.