Well, 10000 BTC is too high, hell even 10 BTC would be rejected. Also if BTC adds sidechain support based on this, people will want it to be
This is the total locked funds for a side chain. You are expecting the bitcoin miners to track your chain, so some spam/DOS protection is needed.
The 100 largest altcoin has a market cap of around $150k. That would be around the 1000 BTC mark.
Starting side chains wouldn't have that much capital though.
Side chains could be recommended to start with a federated peg like Elements. The P2SH refund script could be pay to multi-sig. Once the sidechain activates (>100BTC capital), then the federation would be automatically disbanded.
testnet <-> any standard side chain
or with mainnet.
The point is that elements could demonstrate that it works before adding it to mainnet.