When ETFs with declining assets stop being profitable, the company may decide to close the fund; in general terms, ETFs usually have low profit margins and therefore need several assets to make money. Sometimes, it might not be worth keeping it open. The closing of an ETF is not the end of the world. The fund is liquidated and shareholders are paid in cash.
Often, the ETF earns capital gains during the liquidation process, which will pay registered shareholders and that could mean an unnecessary tax burden. There will also be transaction costs, uneven follow-up, and several other complaints. One fund company even had the nerve to make shareholders bear the legal costs of closing the fund (this is rare, but it happened).