Well, then you are completely free to choose another wallet which guarantee BIP39 support forever more. As pooya87 says, it's highly unlikely Electrum will stop supporting BIP39 since it means all Electrum wallets which use BIP39 suddenly become incompatible. They would be shooting themselves in the foot and alienating a large number of their customers. But even if they did, you can just take your BIP39 seed to another wallet, so no harm done to you individually.

Theoretically yes, but practically impossible, in the same way that address collision is theoretically possible but will never happen.

Even just looking at individual private keys, there are (just short of) 2²⁵⁶ keys. Given that each key is 32 bytes, that works out to somewhere in the region of 3*10⁶⁰ exabytes just to store the private keys. For comparison, there is around 300 exabytes of storage in the entire world right now. So we're off by a factor of around 10 billion trillion trillion trillion trillion just to be able to store the final private keys, even forgetting the multiple keys you have to derive to obtain each one, and forgetting generating addresses, and forgetting all the computing power to do so.