what happens at 1 million users does rc costs per transaction get much more expensive in the RC system

RC costs will skyrocket driving up demand for hive power. It's similar to ethereum gas(fees) basically.

Seems like seeing what's going on with a pool will be important to those delegating to a pool so they can see in realtime what pools may need more attention.

I'm really not sure that people will spend that much time monitoring pools, I expect them to be overflowing with rc as most stakeholders have enough rc to support thousands of users (hundred of thousand for some)

I'm guessing that there will be a few pools with a ton of RC and those pools will delegate left and right, and worst case if the pool starts to be emptied the pool owner can ask for some more delegations from stakeholders.

The key idea for delegation pools isn't to replace rc made from hive power, the idea is to support newcomers so they have a proper experience and then they should eventually either earn or buy hp so they can support themselves.

Rc delegations are also here help account creation teams so they no longer have to delegate hp because delegating hp means you have to deal with all the people who bot accounts because 1000 accounts at 15 hp start to become a nice farm. Now all of those will have 1000 accounts with rc that's worthless so a lot of the services that have to have security like phone verification etc won't need it anymore.

It's actually a json transaction that is then interpreted by the rc plugin:

Here's a few js functions to give you an idea:

function delegate_to_pool(username,wif, pool, amount, ) {
    return new Promise(resolve => {
        steem.broadcast.customJson(
            wif,
            [username], // Required_auths
            [], // Required Posting Auths
            'rc', // Id
            JSON.stringify
            (
                [
                    "delegate_to_pool",
                    {
                        "from_account": username,
                        "to_pool": pool,
                        "amount":
                            {
                                symbol: "VESTS",
                                amount: amount,
                                precision: 6,
                                nai: "@@000000037"
                            }
                    }
                ]
            ),
            function(err, result) {
                if (err !== null) {
                    console.error(err);
                    return resolve(false)
                } else {
                    return resolve(true)
                }
            }
        );
    })
}


function delegate_from_pool(from_pool ,wif, to_account, slot, max_mana) {
    return new Promise(resolve => {
        steem.broadcast.customJson(
            wif,
            [username], // Required_auths
            [], // Required Posting Auths
            'rc', // Id
            JSON.stringify
            (
                [
                    "delegate_drc_from_pool",
                    {
                        "from_pool": from_pool,
                        "to_account": to_account,
                        "asset_symbol": {'nai': "@@000000037", 'decimals': 6},
                        "to_slot": slot,
                        "drc_max_mana": max_mana,
                    }
                ]
            ),
            function(err, result) {
                if (err !== null) {
                    console.error(err);
                    return resolve(false)
                } else {
                    return resolve(true)
                }
            }
        );
    })
}

function set_slot_delegator(from_pool ,to_account, slot, signer, wif) {
    return new Promise(resolve => {
        steem.broadcast.customJson(
            wif,
            [signer], // Required_auths
            [], // Required Posting Auths
            'rc', // Id
            JSON.stringify
            (
                [
                    "set_slot_delegator",
                    {
                        "from_pool": from_pool,
                        "to_account": to_account,
                        "to_slot": slot,
                        "signer": signer,
                    }
                ]
            ),
            function(err, result) {
                if (err !== null) {
                    console.error(err);
                    return resolve(false)
                } else {
                    return resolve(true)
                }
            }
        );
    })
}