A limited fit arena is actually the perfect training ground for learning PvP. The skill ceiling for PvP is very high and requires a huge amount of learning to achieve competence. Limiting the skills to a smaller subset would make learning skills like heat management, manual piloting and maintaining optimal range easier. These skills can then be applied to open world PvP. Believe me - it works.
The proving grounds in their current form have been running for a full year and show no sign of being removed.