Working on this years car makes no sense after Mclaren got this much ahead, teams know that the yare not going to do well, and there is really no reason neither because even sponsors and fans know that they should not expect much from this years car, so no team is excited aside from maybe Mclaren fans.
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Now teams are saving time and money to build cars for next season. Because the more they focus on developing their cars this season, the more time and resources are wasted on developing cars for next season - and also these small improvements may not significantly impact race results, so the constructors think they are just doing what they can, and are more focused on building cars for next season. The only team that I have seen that is making the most noise this season is Red Bull, not about improving their car, but about accusing McLaren of all their wild theories. They can't seem to accept that McLaren is above them.