In terms of income what amount are we looking at in loss if he was forced back on to YouTube ?
Unlike cheers, which DSP gets 100% of (Twitch charges the 40% fee on the user who purchases the bits to cheer with), YouTube takes 30% of all SuperChats he makes. His audience wants to throw $100 at him? DSP is only seeing $70 of that. The minimum you can super chat is $1, and of that DSP will only see 70 cents. All those 50 bitch ears he gets throughout a night that add up won't be there anymore.
YouTube DOES have Memberships, which function very similarly to Subscriptions on Twitch. However, you get a more favorable split on the income here ( 70/30 in your favor, so a $5 Membership works out to be $3.50 to you and $1.50 to YouTube) However, the # of emotes you get are directly related to the amount of Memberships you have on your channel.

Additionally, YouTube wants you to offer something for your Members. For example, a behind the scenes video, or a Members only stream. I do multiplayer games with my Members, for example. That requirement will force DSP out of his comfort zone. It can't be "oh Member's only chat, or a chat badge, or emojis" because those are already offered by the default level at YouTube.
(Must deliver on all perks you offer will be a hard requirement, too)
You can offer Memberships at different prices now, you aren't locked into the 5/10/25 tiers that Twitch forces. You can go as low as $1 for a Membership, or as high as $100.
On YouTube streams you can still run ad breaks to your audience, and monetize the stream to play an ad when someone first tunes in.
But, the tip goal vest streak is all through Streamlabs. DSP will probably push his audience even more to donate through his PayPal/StreamLabs because YouTube takes a 30% cut of everything.
He'd lose gifted subs, he'd lose Prime subs, and he'd lose sub-dollar cheers. He'd probably get more viewers though.
This is correct, no gifted subs, no Amazon Prime freebie subs.
He would absolutely get more viewers. Twitch dominates the streaming market, and is therefore extremely saturated. DSP would have WAY less competition on YouTube, especially if he is able to bring over 300-400 viewers. He might hit YouTube's Live Recommended page.
Plus, as he talks about how shitty Twitch is for only allowing him to stream in 5000 kbps but he "records" in 9000 kbps locally, he wouldn't have to reupload the streams. The VODs will go straight to his channel, he could do "stream only" in OBS which will be less strain on his laptop (not streaming and recording), and he can go as high as 51,000 kbps for YouTube (that's 4k at 60 FPS). Plus he doesn't have to babysit the uploads, retype a title, put in tags or a description, as that's all set when you start the stream.
TLDR: YouTube takes 30% of Membership and Superchats, but I believe a bulk of his earnings is through tips, so it would be a wash. IMO YouTube streaming offers way more services than Twitch and would make DSP's life easier/lazier for his content.
EDIT: DSP averages about ~166 gifted subs a month over the last year, which if they're all Tier 1 comes out to be about $415.63 a month.
DSP averages ~215 Prime subs a month over the last year, which is about $537.29 a month. Losing both of those would be a hit of just under $1000 a month.
If he gets every Tier 1 sub to pay for a $5 Membership on YouTube, he would get about $1003.33 a month, versus the ~$716.67 he gets for all the Tier 1 subs on Twitch. Not enough of an increase to make up for the loss of gifted/prime subs.