There was no downside to the old system in itself, it may have been a bit poorly designed with the granted effects, but the "RNG" based aspect of it was well but together in a way, that you always had a solution left to do and if you were discarding cards, you clearly did something wrong here.

AST would have had great potential as a "high risk, high reward" type of healing Job (or just someone who plays triple triad midcombat) if Squeenix decided too build up more into the old card system, instead of listening to the majority of people who are casuals to the bone who only know the WHM toolkit for healers and view anything else as alien concept that interrupts their "optimisation".

I can only hope that Squeenix will learn one day that focusing on the individuality of Jobs, rather than on the Roles they fulfill, would only improve things for everyone who plays the game, but i guess being narrow-minded seems to be quite the trend in 2020+1.

Also, having Fun should always come first before any optimisation on any part (having 3 WHM Jobs clearly isnt fun in my book), if you think otherwise, maybe you should reconsider if video games really are a thing for you.