You clearly don't drive along the M4 very often
Yes, I know the M4 well. The western rail link will allow people to travel in from the west by train without having to switch and is much needed. The M4 does need widening but I suspect nothing will be done around Heathrow until a decision is made. When you are deciding between Heathrow or Gatwick however, for people coming from that direction it is irrelevant although better to just have to face the M4 to LHR rather than M25 also round to Gatwick.
However you look at it there are some very large and modern rail projects currently being built and we can either build up the airport that is connected to these or one miles away.