If TrainerRoad is a needle in the eye, PeriPedal, PerfPro and GoldenCheetah are too. Those are substitutes for Zwift if you are interested only in erg-mode-like interval workouts (and plans, at least in TrainerRoad and PerfPro). Those will do fine if you strictly want a computer telling you how many watts to put out for how long. Like what people lament about Froomey's style uphill -- do we like seeing him staring at a stem to put out X watts for Y minutes?

In those other training tools there's no scenery, no terrain, no social aspect (outside of going to a PerfPro studio) with riders around the world, no racing or group rides or drafting or accelerating to pass that next rider, or chatting, no sprint jerseys or KOM jerseys or segment PRs, no climbing to the radio tower with Hayman or Boswell or Cavendish, no gamified challenges, etc. etc.