First, there are more than enough improvements. If you are low on infra and have less improvements you must always choose to what improvement you take. If you have arround 2899 infra its high enough to get your first wonder. On that level you have also the most important improvements.
Now many people have -x improvement slots. Its much cheaper and easier buy a wonder than or break the improvements you have and buy border walls, or buy many many infra and then buy border walls. Notify that a border walls also give a -2% citizen penalty. This break issue is also if you add new envoriment improvements.
BTW, if the requirements for the last 4 wonders will be lowered to arround 4000 infra or remove that requirement completely (except for the requirements for the wonders they needed) its also fine, it also handle some problems and issue's.