As I recall, the NEP was already in a troubling position by the time it was historically ended. It has been more than a year since I've studied the history of the USSR, and so my memory could be wrong, but I distinctly remember an economic effect called the "Scissors" I think it had to do with the dependence of the industrial sector on the agricultural sector, and the relative indifference by the latter towards the former. If I remember things correctly, the largely agricultural nature was seen as a real problem in the USSR from the civil war on, and the NEP was at least partially an attempt to guide the agricultural sector towards supporting Russian industrialization. If I have my facts straight, it didn't work as planned by the end. If Lenin lives, which is perhaps doubtful, as some posters here have argued he was syphilitic at the time of his death, and had implicitly been so long before his party's rise to power, then I doubt the NEP would continue indefinitely. Lenin, of all people, is not going to allow the USSR to become a fully capitalist state.
If someone with considerable more knowledge of Russian History thinks I've made a fool of myself, I apologize.