Ion thrusters = technical problem. Thermal rocket = fundamentally limited by rocket equation.
With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do.
Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.
Ion engines are still limited by power density - higher ISPs take quadratically more power input at reasonable ISPs. (As in, at non-relativistic velocities) Thermal rockets have a lower upper bound, but come with higher thrusts. Everything is tradeoffs, in the end.
And why on earth would the kind of newtonial engine mean that you wouldn't be limited by the rocket equation? The only escape that is to have reaction mass outside of your reference frame somehow. (picking up fuel from interstellar space, having thrust beamed to you via laser, some form of reactionless drive...)
No body are talking about plasma rockets (VASIMIR). Elctric rockets that have variable ISP on demand (low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust). Also, VASIMR does not use electrodes so not erosion problem.
With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do.
Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.