True enough. At some point you have to know when to quit, or plan ahead as to where the cost of mods hits the point of diminishing returns.
At some point, with the new power supplies, motor mods, modified or replaced plinths, etc, one wonders where the mod is and why not just build a new machine from scratch?
However, when it was easy to get an SL1200 for $399 (from Musicians Friend), for another $1500 you could add a lightly used or demo SME 309 tonearm, and you would have a $2K turntable that had the tracking *and* the speed consistency and low noise of a more expensive 'table. Rather than put $5K into an SL12x0, however, your money would probably be better spent elsewhere.