The only reason that it's really an issue in an old D&D format is that money earned generates XP. So shopkeepers, merchants, and farmers all earn XP...
If we make the assumption that the D&D GP is the Domesday shilling¹ (1086), a typical laborer makes 1-2 £ per year, or thus 20-40 gp... so a laborer takes 1200/40=30 years to make level 2 as a thief - or if we assume level 0-1 takes as much XP as level 1-2 (I can't get to GHA to check the AD&D rule on that) then to make level 1...
The major farmer would make a few dozen £ per year... 12£ is 240 XP, so 5 years to Thief 1 or 2. The wealthy merchant clearing 100£ per year was getting 2000 XP...
This is where the disconnect hits. Treasure from any source generates XP. Now, if the laborer works only for room, board, and clothing, they get none...
1: There's almost a 1:1 correspondence twixt a report of some prices in shillings from Domesday and the D&D OE GP prices, and I read it in a book I got in 1990 via ILL checked out of the Lake Geneva Public Library with it checked out in 1973 by EGGygax, per the checkout sleeve)... A couple items with mixed shillings and pence were rounded up. Also, a few pages before, mention was made of the gold shilling - 1dwt of gold as a shilling coin. Another reference in that same region notes a 12dwt (silver) shilling... which is, for reference, 1/20 of a troy pound....