I'm a bit late to the party, but I don't think a the factors are quite black-and-white with respect to the certainty of the SLS program. For example, take the patronage of Richard Shelby (R-AL) to the SLS. I doubt the program would be cancelled if his support were to be removed, but I think that there would be some shake-ups. This is evidenced by the fact that support for the SLS isn't universal. A few months ago, the chairman of the Senate budget committee sent a letter to Bridenstine which contained (among other things) criticism of the SLS program. We're all used to treating Shelby as some omnipresent, omnipotent god as far as U.S. spaceflight is concerned (I'm exaggerating just a tad ), but his backing of the SLS program is not guaranteed. It is contingent several factors, including that—
Shelby is alive and well. Shelby is 85 years old. According to actuarial tables provided by the U.S. Social Security Administration, an 85 year old male has (on average) 6 years of life remaining. I do not know the details of Shelby's personal health, but the chance that he dies or is rendered unfit to serve as a senator due to his health is likely non-trivial.
Shelby is elected. If Shelby is alive and fit, then he must be reelected in 2022. Given Alabama's partisan lean and Shelby's own sterling electoral history, I think he is nearly guaranteed reelection.
Shelby is the head of the appropriations committee. If Shelby's party were to lose the Senate, then he would cease to be the chairman of the Senate appropriations committee. This isn't likely given the geographic advantages the Republicans have in Senate elections, but I would still deem it to be a non-trivial possibility.
Supporting the SLS program is to Shelby's benefit. Unless MSFC sprouts wings and flies to another state, this seems almost guaranteed.
(Hopefully I didn't run afoul of Rule 2.2b in discussing all this. I used an example which is political in nature because it is linked so closely to the SLS program, and because it illustrates the similarities in doubt between government backed and commercial ventures. I think I should be in the clear since 2.2b only prohibits content which is political and unrelated to spaceflight.)
Anyway, this is only a single political example. The point I'm trying to make is that there are very real scenarios which cast doubt upon the success of the SLS program: I would not call it a guarantee. (For the record, I do think that SLS has more inertia due to it's government backing, I just also think that it's not guaranteed.)