I did not compare it to V. 

But the discussion with GTA IV was legendary because it would push the consoles so hard,  GTAV had a boost but not that impressive like many expected, it is an recurring discussion.

Sure you did, just pointing out that it's not a relevant comparison. Every year nowadays there are a few game launches that handily surpass GTA IV's in scope (Palworld, Black Myth, Helldivers, MH Wilds, etc.).

Modern GTA is a completely different beast than it was even when V launched, the only thing it compares to are Minecraft and the largest free-to-plays, really. This is all assuming, of course, that VI comes with an online mode.

Edit - that's not to say, obviously, there will be millions of extra hardware units being sold in May or June of next year. But it should help the late years of either console more than, well, anything else that could have released in these platforms, really.

Probably about 4-6 million units to the final numbers of Xbox and PlayStation.
Probably not more than 1-2 million units added in the launch month (even if it's the second half of May and the first half of June).
It will probably only add about 1 million units lifetime to Xbox. The rest will go to PS5.

There will be a boost the month it comes out but it won't be that huge due to how late in the generation its launching. RDR2 didn't really cause a boost at all when it came out and GTA 6 will be releasing even later than that did, especially if it gets delayed into the 2nd half of 2026. There will still be an impact since GTA 6 hype is astronomical but it's more gonna be felt this year and last year than in 2026 I think. I bet a notable amount of PS5's sold late last year were due to GTA 6 and the same will be true for the holiday season this year. Hell that'll be going on to a degree right now since if someone intends to get a PS5 for it they may as well get one now before the price likely goes up.

It'll also benefit Xbox since the Series S will be the cheapest way to play it but with Xbox hardware dying it won't benefit as much since a PS5 is a clear better buy for the vast majority of people.

For PS5 I don't really see it doing a lot, unless this gen is going to last until 2030, but the rumors are 2027-2028.
My friends and I have basically decided to just wait until PS6. One buddy will cave to play GTAVI but will definitely find the cheapest used PS5 he can. As far as the rest of us are concerned, there's no point in buying a PS5 later in the gen if the price is just going to remain this high (or go even higher yet). It makes way more sense to just get a PS6 in the launch year. PS6 can't be more than $599, and if SNY is smart they won't go over $499, even if that means only a minor improvement over PS5 Pro specs. That way we aren't stuck with a last gen console, and have the newest gen for the same PS5 price, or not much more.
I think this will be true for quite a few people, which makes me believe GTAVI will help boost PS6 sales early on, just as much or more than it boosts late gen PS5 sales.

Not sure what MS is planning exactly, whether they'll make sure they've got plenty of Series S consoles available, or if they're pushing to have next gen ready by holiday 2026, but either of those scenario's would have me believe that GTAVI will boost XB console sales quite a bit.
I also can't help but wonder if the May 2026 GTAVI launch date very well may be due to new XB hardware holiday 2026. Next May would pretty much be splitting the distance between holiday 2025 and holiday 2026. As long as MS were to announce that before May 2026, it would absolutely help boost those holiday XB console launch sales. Rockstar may also prefer that because it's possible that GTAVI very well may not play that well on a Series S, and Rockstar would probably prefer XB gamers get the game on brand new state of the art hardware, where GTAVI will be able to shine.
Even if there is no new dedicated XB hardware, then I think as long as MS makes sure there's plenty of Series S consoles, they'll sell far better than some may anticipate, simply due to being the cheapest next gen console available, assuming SNY increases PS5 pricing again. However, if SNY holds out and keeps PS5 pricing as is, then GTAVI likely won't do much for Series S sales.

I really hope it does last that long.  This gen had such a false start, thanks to covid, I think a few more years is needed to flesh it out

I think it will be the kind of hyped event that attracts extremely casual gamers who otherwise would be happy to check out of the generation (honestly I think Hogwarts is similar but on a smaller scale). I'll say 10m

I think it will but next gen will be there as well. Developers are only just finding their foot with a lot of new engine technologies, I think PS6 for a while will just be PS5 games with all the bells and whistles. 

I'm not so sure about that since by the time it launches $499 in 2020 will probably be over $650 and perhaps even a bit close to $700 if it's 2028 so I think it costing the same amount as the PS5 Pro does currently is likely unless it's underpowered since $599 would probably be the price point for that instead of $499.

I am willing to beat just 0.5 million more. It's coming out pretty late in the consoles life.