I have some sympathy with the "ruining old bikes" argument.
But mainly because, around here, clueless hipster fascionistas are busy turning nice old road bikes into neon-coordinated fixed-gear-freestyle monstrosities. ****S!
I've had 4 singlespeeds. 3 already had track-ends or EBB; 1 was a conversion with Paul Comp dropouts. But that one had previously cracked at the dropout (that old Kona favorite), so I basically rescued, not ruined it.
SS can be great. It's like the simplicity of your first BMX all over again:
Pedal to go;
Pedal faster to go more;
Stand and pedal harder to go up;
Brake to stop;
Smile;
Repeat.
No cursing as your different groupsets don't work together;
No replacing crazy expensive cogs, rings and chains after 1 winter;
etc.
etc.
But it's not for everyone.
And some people are overly-enthusiastic about the whole singlespeed niche thing; They need to get a life. It's just another bike. Albeit a fun type.