I find the premise of your suggestion to be false. Increasing TVL just for the sake of increasing TVL is not useful. The TVL increases, if people want to use the stablecoin. It is not sustainable to bloat the TVL beyond what is needed for usage.
You are correct in that RSV holders don’t have good enough incentives. Though judging from current reserve ratio (487%) there is still enough people who hold RSV anyway. The incentives can improve by more usage, or changes. Sigmausd is not abandoned, the usage/profitability is being monitored, while the changes are discussed. You can search my username for proposals on the topic.
Your suggestion to change the fee structure is good to consider. It should be added to the SIP discussion. One consequence would be that arbitrage would behave differently via Ergodex. If fees are balanced, then 2% difference in price in either direction is enough to make arbitrage profitable. In your suggestion this dynamic would change. This means going off peg more than 2%. Not sure if this is desirable or not, just something to keep in mind.
Dexy will not replace sigmausd. There will be different stablecoin experiments, and that is good for the system as a whole. Time will show which ones are sustainable.