If a broker offers online trading only for the smartphone, then this should give reason for raising an eyebrow... Something is not kosher with such a broker... Ie. there is a hidden agenda behind it...
I agree that broker requiring ONLY phone should have hidden reason. Similarly if some home cam company offers free use, then the company may sell personal shot at bathroom from you, to others with profit.
I love ET conspiracy theories. Please enlighten me what is not kosher or what the angle is. There is a whole demographic that increasingly only use smartphone apps and don't want or need a desktop app equivalent. Perhaps you aren't the target demographic for these brokers. I suppose you would suggest the same conspiracy about the new challenger banks that are also mobile-first? eg. Mondo: https://getmondo.co.uk/ I think a more likely explanation is that this is what the future will look like across the board not only for banking and trading but for all kinds of verticals.
I agree that phone is too weak to penetrate, than web or PC app. I hope broker may make a NEW CLOUD MACHINE for security of traders, so that nothing is installed/running except the trading software. Of course we see their screen with our end PC/phone at home. As we know, there are already lot of (almost-free) cloud machine, such as AWS(amazon) and others.