Steve Job's stated reason switching to Intel is that Intel had developed a process for producing cool chips while IBM was not interested in producing processors that were suitable for consumer products. This is the reason that Apple was never able to produce a PowerBook G5. Intel's cool chip manufacturing process was a surprise based on its history going back to the 386. The 386 ran hot. For those who remember, IBM was late to the market with 386-based systems because it had trouble managing the heat generated by the 386. This helped to end IBM's domination of the IBM-compatible personal computer space.
Saying "PPC really fell behind" is a very broad statement. Where it fell behind was in the area of heat production. It did not so much fall behind as IBM has always thought of itself as the producer of Big Iron. It did not concentrate on designing processors suitable for the consumer market. After Jobs announced the switch to Intel, then IBM suddenly saw the error of its ways. By then, it was too late.
However, heat production is not the only system metric. Having used a number of Intel-based systems going back to the 1980s and more recent systems going back to 2007 or so to present and having used PPC systems going back to 1994, I can state without fear of contradiction that PPC-based systems are much easier to live with than Intel. By this, I mean that my PPC-based Macs allow me to get my work done without hassle. I find the difference is subtle, but Intel-based Macs do not age as well as PPC-based Macs.