In a vacuum, Bane’s idea would make sense. However, in actual practice, it didn’t guarantee superior power and strength. Bane was not omniscient, and treating ALL rule of two Sith as inherently greater than all who came before is very very flawed. Just because Bane meant for that to happen doesn’t mean it did. Sidious coming along was a godsend for the Sith. Without him, Plagueis probably would have gone for Dooku, if not just abandoned having an apprentice entirely (he only took Palpatine because he saw it as a spur of the moment gift from the dark side). And good luck to Dooku being able to fend off Yoda. 

Bane vs Zannah: Zannah won due to an amp from the planet Ambria, a dark side nexus which allowed her to create tendrils she otherwise could not have. Even if she did end up being stronger than Bane later in life, actually fighting him didn’t make or prove her to be stronger.

Gravid vs Gean: Sure, Gean did manage to beat him, but he destroyed so many Sith texts that he pushed the grand plan back by centuries. So anyone coming after Gean is going to have the disadvantage of not having access to all those ancient materials. Even Plagueis and Sidious didn’t rediscover them by TPM.

Tenebrous vs Plagueis: Killing your master while he is distracted trying to save your life is not an indication that you are more powerful. Plus Tenebrous actively sabotaged Plagueis and sought to end the rule of two with him. The Sith got very lucky that Plagueis managed to get as strong as he did in spite of Tenebrous endangering the grand plan.

Plagueis vs Sidious: Yes, Sidious ended up being way stronger, but him killing Plagueis on a spur of the moment decision after getting him wasted and defenseless does not subscribe to Bane’s ideal. At most, every subsequent Sith becomes a better cunning opportunist, which is important, but it doesn’t guarantee raw power at all.

Overall the Rule of Two doesn’t guarantee the apprentice will exceed the master. It is overrated, flawed, and even its practitioners post Bane did not obey it fully and faithfully. Without Sidious existing, no darksider would have had the power to take over the Republic and kill Jedi top tiers as he did. Sidious’ strength came from his own natural power and dedication, Plagueis’ training didn’t guarantee he would become stronger than him and he certainly didn’t surpass Plagueis the “correct way.”

The Rule of Two doesn’t guarantee a stronger Sith. It is inherently flawed and it got lucky with Palpatine at the end.