Taking your post as a question rather than a statement, the answer to "What if?" is "they'd get smashed".
There's an idea abroad that coaches are clogging up the game because of some aesthetic predilection. Of course that's bunkum; they do it in response to their universal two-word KPI: Win Games.
Do this: get two monitors, play two games at the same time, one from this year and one from any time in the past you choose, and imagine one of the teams in one game playing one of the teams in the other. Carnage.
That's why the arguments about reducing interchange, or abolishing it completely and going back to the old reserves, are rubbish. Coaches know that winning happens because you get bodies around the ball and bodies where the ball is most likely to go. They'll just recruit and train endurance runners so they can continue that strategy. Our game is fundamentally different now because coaches and players have the skills and training to implement strategies that increase the chance of winning - whether we like how it's done or not.
(Oh, and while you're watching the modern game and the old one, you'll quickly see that all the bleating about how poorly players kick these days and how marvellous they once were is backwards-looking-through-rose-coloured-glasses hogwash.)