I'm just going off of what the viewer was shown. Gus wanted 3 months of Walt's time. What happened after that 3 months? We don't know. Was Hank going to stop looking for who was behind the blue meth in 3 month?
It's not complicated. He wanted his product. He employed him to produce his product, after 3 months was Gus going to stop his operation? Or was he going to replace Walt and keep it moving?
Gale was going to be his cook. He just had to speed up the process of him learning Walt's method sooner because of how crazy Walt was. Not complicated at all.
After the 3 months he would've likely just brought in Gale, Walt's original assistant to continue running things. Remember, at that point, Walt wanted out the game. Gus would've had no reason to kill Walt.
The original plan would've had Walt working with Gale. Gale learning the recipe, Walt getting his 3M after 3 months and retiring from the game. Walt dying was not a requirement.
That changed when Walt convinced Gus to get rid of Gale and bring in Jesse. That changed when Walt killed two of Gus' men.
Before Walt killed two of Gus' men, Walt proposed the extension for 15M a year. Gus accepted.
Gus didn't make plans to get rid of Walt until after two of his men were killed.