What teams can prepare for a new season without buying players is two folds. They could train better, see the videos from last season and see what mistakes were made and try to cover their weaknesses and then train accordingly to be better at what went wrong last year.
And secondly, they could also try to figure out a way to get "fatigue" injuries lower, which means they need to make sure that the players are not tired all the time, they can check their schedule, they can check their team squad and depth, and they can try to arrange a way for players to replace each other, even out of position changes could happen if you could give the players enough time to train for it. Obviously every team also gets new players and even sell some players, but these other two things are very important too.
your opinion is indeed very good and it is true of course by doing things like that such as practicing hard and also learning from experience it will certainly have the potential to make a team even better. and indeed if the method you mentioned for example is successful of course it will certainly make a team cost-effective and of course it will make the team far from the word loss as long as the team can really win the trophy.
but in my opinion of course strong new players are really needed because sometimes there are times when everyone has different strengths and of course not everyone can train then their performance will increase rapidly and of course in a club most of them all train hard but of course the results of the training obtained by each individual are different. so it's no wonder so many teams buy strong players and finally get satisfactory results. but there are also those who train hard but have not produced anything positive. but in my opinion of course for a team to achieve success of course there must be many strong ones and
if necessary to bring in strong players then of course a team will be stronger.