Just as a heads up, this isn't really true. Perhaps you could make an argument in 2.X, since your total number of abilities would be the same (10 Cross Class vs. 5 Cross Class & 5 Job), but even then the stat bonuses and powerful actions more than made up for it in the vast majority of cases. There were some PVP Maurader and Archer builds that were arguably better, but even then it was only for PVP. Post Heavensward, you're missing 5 total abilities, and additional Stat bonuses. It's technically "viable" in that you can complete content, but at a significantly higher difficulty for not just yourself, but your entire party. In many cases, you're missing out on additional core mechanics post-heavensward. Warriors lose access to Deliverance (and Defiance), Paladins lose their Oaths and Royal Authority/Goring Blade, Black Mages lose out on Fire IV and enochian, Dragoons lose jumps and Blood of the Dragon, etc. You're losing so much potential DPS, Healing, and Damage Mitigation (or all three, depending on role) in exchange for some non-trait enhanced Cross Class abilities that are never going to make up the difference. It's fair to wish they were as viable as Jobs, but Classes are slowly being phased out. They're revamping the "Cross Class" system into a role based ability system, and even then all new Jobs thus far haven't had corresponding classes (Dark Knight, Astrologian, Machinist, Red Mage, Samurai). Once we hit Stormblood, you'll me losing even more total usable abilities. Whether you like it or not, Classes are not meant to stay viable.
Please don't spread the "Classes are as viable as Jobs!" myth any further. It just confuses new players unnecessarily. Once again, you can complete content using a class instead of a Job in high level content, but you're drastically increasing the difficulty for not only yourself, but your entire party. If you're just playing with Friends, do whatever you like, but when in DF, just put on your Job Stone.