he and sengoku couldnt even hold pre ts blackbeard properly let alone holds bm and kaido at same time. back time ace cannot touch wb in his sleep due to greatness of WB advanced coa, but later squardo and all those marine fodders easily can stab and shoot wb. same like pre ts G2 can make an open wound in garp's head at marineford war.these ppl have changed alot by time.
38 years ago roger and garp beat rocks yeah true fact, but doesnt mean that thing never change to the end, later WB ( who ppl claimed suffers defeat by garp and roger at valley end, just like kaido and linlin ) fought roger countless time on equal ground, and roger himself avoid to confront BM. that's all because some people are grow stronger while some of them being old and weaker. rocks only groups that have incompetent team of works and wb already said that each of them hate each other.
just like what sengoku said. if bmp and bp formed alliance, it will be the formation of worst pirate crew, means this alliance is way dangerous/stronger than rocks pirate itself. even akainu say the balance of the world will tip to the pirate side if bm and kaido allied.means all those shichi/SSG and marines are meant to balance 1 emperor not two let alone four. since each emperor rival/enemy to the other and doesnt work together unlike marine wg and shichibukai.