A lot of the comments above are right on target -
Maltodextrins have high GIs, because they’re mostly glucose. They don’t quite qualify as a complex carb; the simplest maltodextrin is two glucose molecules together, which can get absorbed faster than straight glucose - that’s why some maltodextrins have a GI higher than 100. Pure glucose has a GI of exactly 100 (it’s the standard they calibrate against); molecules of maltodextrin are absorbed more slowly than molecules of glucose, but each molecule contains at least two glucoses, so it can end up raising your blood glucose more than straight glucose!
Starch is more of a complex carb - and maltodextrin is just starch broken down to be simpler. It’s broken down almost enough to make it into dextrose, a simple sugar.
This also plays into why it’s a good ingredient for a shake - like a starch, maltodextrin still thickens a liquid, and has a pleasant mouth feel, without excessive taste. Recall that they were NOT trying to make Soylent sweet.
Sugar has a syrupy mouth feel, and it definitely makes things sweet. In large quantity, it also starts getting hard to dissolve (think sugar granules at the bottom of an iced tea.) The sweetness would probably stop you adding any more sugar long before the mix became syrupy, but that’s the direction it would go - whereas maltodextrin adds a nice creaminess, and still dissolves easily in cold water at fairly high concentrations.
The sucralose in Soylent is not for sweetness; they’re not using enough of it, for that. The small dose of sucralose they use is primarily because it’s effective at masking other flavors - I think especially the bitters/tarts of their multivitamins. I’m speculating on what flavors they want to mask, but I’m quite sure they’re using sucralose to mask, not to sweeten.
They probably also had some desire to avoid the bad press that comes with “sugar” on the label, given the amount of bad press sugar and fructose get. I completely disagree with the bad press; I think fructose-mongering is 99% misinformation, but it can still affect your product badly, so if you’re not forced to include it, why include it?
If you want to make your DIY sweet, there’s no reason not to use sugar instead of another sweetener. It’s a perfectly healthy choice of quick carbs, as long as you limit it to a healthy proportion (you don’t want a ton of quick carbs in your diet, but there’s nothing wrong with them in moderation, unless you have a carb-management dysfunction, like diabetes.) Sugar in your DIY is certainly no worse for you than equivalent calories of maltodextrin. Neither is a good source of anything except simple calories, so no harm, no foul.
Strictly speaking, if you take out 100 calories of maltodextrin and replace it with 100 calories of sugar, you’ll make the end result sweeter, and you’ll lower the GI value, which is a good thing. Depending on how much sugar that is, you may find it takes a while to dissolve if you use ice water to mix.
Good luck, and have fun experimenting!