I'm a bit confused about what you're saying here.My worry with TVMs is (a) that they won't work and nobody will believe me and (b) if I've cut it a bit fine and there is someone already at the TVM who can't work out how to use it and my train is already at the platform.
You need to arrive at the station in good time to purchase a ticket. If the TVM is working and accepts your chosen method of payment then you must use it to buy a ticket. Someone else already using the TVM who is taking their time and your train being at the platform doesn't change this.