Is the guru-disciple relationship eternal?

Question: Is the guru-disciple relationship eternal? If yes, then what happens when the guru has some difficulties in his devotional service and resigns from the service of the guru?

Some important quotes are given below:

The guru is not just a person but a principle

“Spiritual master is not that a particular man is spiritual master. Spiritual master is a truth. So what is that truth? The truth is samsara-davanala-lidha-loka-tranaya karunya-ghanaghanatvam **. The whole world is in the blaze of material pangs, threefold miseries. And a person who is authorized to deliver people from that material pangs, he is called spiritual master.”

 Sri Vyasa-puja — Hamburg, September 5, 1969

 “As God is one, similarly, guru is also one. There cannot be different gurus. Because God is one, how there can be different gurus? The principle of guru is one. The original guru is Krsna.”

 Arrival Lecture — San Francisco, July 15, 1975

“guru cannot be two. Mind that. The person may be different, but the message is the same. Therefore guru is one.

 The Vedic instruction is, tad vijnanartham sa gurum evabhigacchet [MU 1.2.12]. Gurum eva, “one must.” Eva means “must.” Abhigacchet, this verb is used when there is the sense of “must.” It never says “Go to a guru,” but he says “Must approach the guru.” Guru is one. Guru cannot be two. Gurum evabhigacchet. And we see also, practically, in the disciplic succession of guru, the same thing is spoken by the guru. Same thing. Repetition of the same subject matter, no other.”

Lecture: What is a Guru? — London, August 22, 1973