There's no actual harm in draining your pasta water into the sink.
I have drained pasta water into the sink and then run water down the sink to clean it and wash it down the drain.
The reason you're not supposed to drain pasta in your sink is because the water from the pasta helps to make the perfect sauce in your pasta.
When the pasta water is mixed with the sauce it makes the sauce thicker and much better and helps the pasta sauce stick to the pasta.
So the real reason you should never drain your pasta in your sink is not because it will clog your sink but because the starchy, salty water is the perfect emulsifier and thickener for making a spectacular, silky sauce.
The starch gets into that water when cooking the pasta.
And that starch helps make the perfect pasta and sauce thickener.