The counter argument would be that similar attractions were a success at Anfield and at thr Tottenham stadium. Then again, those two stadiums host anywhere between 20-30 well attended home fixtures every year, versus the 5-6 that the WRU host, plus concerts.
Tottenham probably had in from the design stage a roof walk which would have meant a lower cost. If the roof walk was £5m then a massive chunk of that would have been design, construction and modifications required to enable safe access for untrained people.
The hope was that people would pay top dollar for the chance to be in the stadium without having to endure us playing.