Rolex's innovations with the 904L steel are mainly dealing with case parts and it's really unnecessary IMO. Just look at old vintage oysters that has little corrosion and pitting under proper care.
The "It takes a year to make a Rolex" has been their advertising slogan since the 1950s. With all these technological advances, they've managed to increase the time it takes to make a movement to a mere 14 months with more automation and robots. The finishing of their movements are automated, which negates any effect of it being "in house". Collectors who care about watch movements prefer movements that are finished by hand, i.e. things which robots cannot replicate. The basic design of a watch movement hasn't changed much in the past 50 years anyways so the "in house" branding is just really a marketing scheme. And I know for sure that it takes less than 14 months for Philippe Dufour to manufacture a movement from scratch. That's why it makes more sense to go with Rolex frankens rather than going for gens.