Test format is depleting already and we only have very few teams that can match or even capable of playing 5 long days and still makes it interesting. That is one of the reason England is doing something called bazball even if it is not giving them the results they wanted while other teams like AUS and IND makes it aggressive also a balanced approach.
If we only keep these best teams and remove others then the potential viewers will reduce that affects the revenue and ultimately every sport is a business so they will try to keep it stretch as long as possible while concentrating on the profit making format that is why T20 is given more importance than others.
Teams like England with their Bazball strategy and other aggressive teams like Australia and India are trying to make game more dynamic to attract viewers. However because cricket is business shorter formats like T20 are more popular and make more money. This puts cricketing authorities in tough spot they need to make Test cricket both interesting to watch and financially successful without losing its special qualities all while making sure sport continues to grow worldwide. It is difficult balancing act that will determine future of format.