Indeed, but the set of aerodromes that are large enough to have a tower controller but not large enough to have their own radar surveillance is shrinking all th

Indeed, but the set of aerodromes that are large enough to have a tower controller but not large enough to have their own radar surveillance is shrinking all the time. Radar is getting cheaper and what with ADS-C and TA/RA, a big reason to have ATC even without radar is vanishing (namely that of preventing collisions close to the airport). Oceanic control is probably the closest you can get nowadays to routine ATC without radar, even though they now have automatic position reports via satellite.