Depending on where this goes I have a theory. If this is truly the end of Ippo then we can draw some interesting comparisons.
The decline of Japanese masculinity as a whole and the desire to recreate the Samurai logos.
Ippo represents the Japanese people. Not the biggest guy in the world, but strong with an indomitable spirit. He is guided with the help of the Coach to understand himself, seeking to understand what it means to be strong. His own father died a hero, and so Ippo strives on the one hand to understand his father's courage and 'fill' his father's shoes and on the other to understand his own limitations (and in this case breaking himself in the process).
Japan is known for overworking their people causing them to break from stress, illness, and other work-related problems. Kurosawa represents, in essence, the elder Japan, the government, or even Japanese authority (like the Emperor). He cares for Ippo deeply, but is so blinded by what was (Ippo's past performances) and his own desires that he fails to properly assess present Ippo and we have a tumbling, bumbling Ippo in the ring getting knocked down by a graze.
The same goes for the Japan of today. The herbivore men and so on. The authority, if we may use the term in this sense, pushes the population into ever more difficult and precarious situations without properly understanding the status of the current population, which thus results in the current population decline and overall degeneracy of certain aspects of Japanese life. I am no expert on Nihon, but I believe this analysis holds true.
There is a strong desire for a return to the way of the Samurai, the old ways (Kurosawa's own admission at the start of Hajime was that Ippo was a fighter in the old style, and the constant references to the Samurai). To understand honor and strength, but the population is overwhelmed with its own dilemmas and an ever weakening society struggles to meet this demand. So Japanese men withdraw into NEEThood or into the Hikikomori lifestyle. Society has no place for them, so they indulge themselves, finding meaning in simply existing (which unfortunately, is no meaning at all). This all goes back to WW2. Japan is as much a victim of that war, losing much of their best men and people. So, Ippo offers a possible return to the old ways, but as much as he tries and as deeply as Kurosawa cares, they are doomed because they want to recreate something that has died and hurts that which has potential to grow.
The Dempsey roll is an incredibly dangerous boxing maneuver that is to be supplemented by clever technique. Something which Ippo lacks. Taking the punishment that he has and still fight is nearly impossible. That's why at the age of 21(or 23), Ippo is washed up. In the same way, Japan is trying to brute force their traditions into modern society which creating an inhospitable environment for a healthy society. Instead of adapting and moving forward with their traditions, they cling too strongly and so cause strife in the population and the possible eventual failure.
Note: This is not to say that Japan is entirely at risk, this is merely a possible comparison that can be made between the two. Japan has actually done quite well in remaining homogenous and true to their traditions, but the danger of multiculturalism and degeneracies within their own society could be disastrous in their future, as it now is for Ippo. This is a pretty sad ending to a classic.