Everyone is beatable. Robinson or Ali on their best day would still fall victim to a bad style match up, because they still had weaknesses.
He was considered unbeatable against the competition available at the time, especially after the Williams, Terrell, and Folley wins. There were still questions about how well he could take a punch, how he would hold up under a fighter who could punch and keep pressure on him. Rocky Marciano voiced these questions during a 1966 interview which is available on YouTube, and on the Clay-Jeffries computerized dream fight (also on YouTube) the question is asked "How well can he take a punch?" Some were calling him an all-time great at that time, but it wasn't until the FOTC that any lingering questions of his gameness and durability were answered conclusively. Another measure of how he was perceived at the time would be in the betting odds of his fights. He was a heavy underdog against Liston in their first fight, and remained a slight underdog in their Lewiston rematch. He was favored over Patterson, and was usually a heavy favorite over everyone else through '66 and '67. (Ring Record books carried the actual figures.) Ali was favored over Quarry and Bonavena when he came back in 1970, and was a 6-5 underdog against Frazier in FOTC.
Anybody can be beat of course, though at the time there was no competition to really threaten late '60's Ali. Even when he did not do great like against Chuvalo & Mildenburger, he won handily, count the rounds, it was not close. Though the quality of his competition could easily be underrated. Like the vast majority of the time in HW history it was not great, but it was not bad. And someone like Foley: it is deceptive to say he was only .500 for the rest of his career. First, he was 5-3-2, & won a Ko & TKO in his 2 post-Ali fights. And how about saying he had not lost for ~ 4 years, on an 11-0-1 run, before Ali, beating Mildenburger, Foster, Bonavena, Chivalo...Machen, Jones & Cooper earlier. He was a legitimate contender. He just got old several fights after Ali.
Of course not. If there was a fantasy round robin tournament of the best 50 heavyweights, all prime, where everyone fought each other once, any number of guys could come out on top. Nobody's going 49-0 in it, though.
Career timing has a lot to do with how invincible a fighter looks. Nobody fought everyone on their best night and nobody produced their apex performance each time. Generally speaking, if a fighter is evenly matched on his own super level he wins only half his fights. A fighter looks untouchable because there isn't anyone on his level ...in that moment in time. There have been special fighters who produced a lot of special consistency. I think Joe Louis outclassed a lot of good fighters, however, possibly only Baer was at his best. Walcott was at his best but he was not outclassed like Baer was. Ali outclassed Patterson but Liston was inactive. Foreman wiped out Norton, that was good. Ingo wiped out Patterson and machen. Foreman and ingo did not produce the consistency however. Marciano had consistency but he never looked unbeatable. just hard to beat. besides nobody likes how old Rockys foes were in the biggest scheme things considered ATG. Lewis did not outclass anyone considered the world's best or the world's next best. Nor Holmes. Tyson outclassed Spinks and a lot of inactive faded guys, many who were not beating rated opposition. No. I don't think anyone was invincible. Lots looked invincible for a time.
Tyson. He looked as close to a complete fighter as can be. He didnt have his jaw broken and didnt suffer any flash knockdowns, and you can count on one hand how many people actually landed a significant punch against him at his peak.
Not even Clay can compare to this. Cooper decked him pretty hard in their first outing. Nobody is unbeatable, but Tyson looked damn near close in the 80's.
Id argue that Tysons pre prison opposition was better than pre exile Alis, and he looked better in beating them as well. Obviously what seperates them is what happened after their respective exiles.
Ali from 64 - 67 was considered unbeatable. No one could work out how the guy has so much speed of both hand and foot, coupled with tremendous athleticism . It was that simple then and it is still that simple now, because it has never been seen before or since in a Heavyweight. Pleased don't even think of mentioning Roy fvcking Jones and his one foray into the division.
If you put a certain rule set in, you can have a man who's going to win 90% of the time. There is no such thing as unbeatable.
If we were to take all of the ATG's and put them in a ring with each other prime for prime the win-loss-draw columns may look like 3-3-3 and that is included Ali, remember he had a scare against Henry, Banks and Jones and lost to a prime FRAZIER, Louis Marciano Lewis Dempsey etc. would all have losses but nothing to be embarrassed by I think most champions built their records on the bones of ex-champs Louis may hold the record and Tyson up there as well as the rest-Johnson,Jeffries,Marciano,Lewis Prime to Prime is a horse of a different color
Even THE great man could lose on a given night. Nobody's absolutely unbeatable. Even the best fighters are human