Sure, HIVE keeps working (and will do so), but also neither attracts much attention from outside nor really lots of new (longterm) users.

IF splinterlands players would power down their SPS to invest into hive, that would crash the splinterlands economy ...

Well, there is nothing much to crash anymore. :)
However, the altogether value of SPS in USD still shouldn't be underestimated.
If more clever they should have started the conversion process of SPS into HIVE quite some time before launching the proposal to get a better price when selling it slowly.

... and send hive ...

... to heaven? :)

Yes, the liquity is so low that everbody who would buy huge amounts of HIVE within a short time span would increase the price significantly (and make the rich HIVE whales even richer).

I am personally rather neutral in this matter and have no clear opionion what might be best for Splinterlands. If Splinterlands won't get funded from HIVE but get real support from another chain, maybe it will be worth a try to change (before dying anyway). A successful Splinterlands on the otherside might be benefitial for HIVE, too (maybe should then stronger encourage their users to use HIVE also apart from playing Splinterlands only).