You need more than just a tune for reliability and real power. Sure you can get a cheap boost but everyone is going to realize they need to add hard parts like downpipes and intercoolers. Then you get into drivetrain issues.

And you're talking about a very, very small group compared to the old days when the hobby was more popular. Honda Civica and Ford Mustangs were such a hugely popular vehicle to modify.

It’s all a matter of the vehicle and what your goals are- sure at some point you need to modify everything else, but a turbo car gets a lot further to start with for a lot less money. I grew up tuning old Volvos and Mercedes and they were so well made and undertuned from the factory you could just about double the horsepower for next to nothing and they were still reliable. Early on they even used massively oversized turbos that had a ton of lag but could really have the boost cranked up. European cars, especially older ones tended to be heavily overengineered - with the drivetrain, cooling system, suspension, brakes, etc. massively oversized compared to Japanese and American cars.