But I guess I will never have such big amount of bitcoins, although I want to, so I will be not having such problems.
you are right, blockchain wallet is one of the trusted bitcoin web wallet and it is indeed that keeping our coin in to different wallets especially if we are owning a huge amount of bitcoin could be good but I think, to be secured it is much better to use a cold storage whenever you are going to store a large number of bitcoin .
100 bitcoins is a lot of money. Im sure someone trusting bitcoin that much wouldn't have a big problem downlaoding bitocin core and running a full node locally to guarantee that they are validating transactions correctly aren't they? I mean i dont get the people that invest so much money on bitcoin and can't even take the time to download the full blockchain. It's still only 100 GB, pretty sure if you are holding 100 BTC you can have a hard drive to host the blockchain. Please let's not be lazy with this and run as many bitcoin core nodes as possible, this helps everyone invested in bitcoin. Leave electrum for smaller amounts imo.