That most Splinterlands players don't invest in HIVE doesn't necessarily mean that HIVE isn't perceived as 'Splinterlands chain' from outside of the ecosystem.
One criteria could be who causes the majority of transactions on the HIVE blockchain.
Another would be to check if search engines like Google show more results concerning Splinterlands or HIVE.
Interesting would also be to compare the value of all assets (including their HIVE, but not only HIVE) in USD hold by Splinterlands players compared to the HIVE value in USD hold by non-Splinterlands players to decide which ecosystem is bigger altogether.

I think IF Splinterlands player would power down their SPS reserves to buy HIVE they would easily be able to let the proposal pass. If they will do that only time can tell (it lasts four weeks to power down 100 % of the SPS stake).