I guess there are lots of 15s, 66s and 201s around that can do a superb job in 2020. I have used Gütermann top stitch and extra strength threads in my machines, and slightly thicker threads of other brands with out problems. I haven't noticed any overload on my 201, it sews most materials nice and easy. Sewing layers of upholstery I have had tension problems but it was easily solved. I think these machines were made to take larger needle sizes. The last pages of my 201 manual shows a needle/thread indicating chart and it looks like Singer once made size 20 and 22 needles. I could only find them in a Chinese brand, but they did the job. Getting the larger shaft and groove in the neede solved any tension issues.
I can't recommend pushing these old mahcines beyond their limit, but they happily sew most types of upholstery, light polyester sail canvas, even bull denim (within reason). Anything that easily slide under the presser foo ist generally fine. You notice when motor and machine start to struggle and I avoid it. Among domestic machines, the old cast iron straight stitchers are in a league of their own.