Couple things:
If Steem were to get listed on Coinbase the benefit would be that people would have a direct fiat on/off ramp in the US. Currently there are none.
People would not be holding their steem on Coinbase, just using to get steem in and out of steemit or any other app on steem.
In regards to holding steem on steemit, if steemit were to go down you could always access your coins through a different front end as long as you have your private keys, so I don't necessarily share your concerns about needing a smartphone app wallet.
Though, overall, it would be a net positive to have one.
I don’t disagree with your thoughts dude.
IMHO we are a while away from a mass adoption of crypto currencies, when that happens the non institutional less tech savvy investors are going to need simplicity. I bought Steem through a Freewallet app about 6 months before I joined the platform, since then Freewallet have dropped Steem which I saw as a lack of confidence in the token.
The easier a coin is to buy and hold in your own possession for a crypto novice the more likely they are to invest.
FYI I’m not advocating for Freewallet, I think they’re dodgy.
Oh yea more wallets would always be nice. Though, most likely them dropping it had to do with the cost of hosting a steem wallet. That still needs to come way down and is part of the reason exchanges haven't been jumping up and down to add steem. That or most steem users being on steemit could also have been part of it :)