Should I ditch my Mch for SAM or a caster?

[Saefinn]

You can clear any content as MCH, so the only true rule is to play the job you like the most.

The difference between SAM and MCH is that you won't be the game changer if your team needs more DPS, also expect to have ranged specific mechanics to deal with and Tactician to use. That's about it.
Your DPS only matters in content where you can't rely on your teammates DPS, think about EX/Savage with pugs. Since DPS is a taboo in those content, the easiest solution is switch to a job that brings what the team lacks the most.

I usually play MCH with my group but switch to SAM when farming EX as it is more reliable and no matter how hard I'll fail as a SAM, I'll still pull more DPS than if I play my MCH at a near perfect level. And you have neat role skills such as Bloodbath and feint if your healers are struggling.

Otherwise, I'm playing MCH with my group as I can trust the DPS and the healing.

Heck, even then, it's fine for EX and Savage.

[CKNovel]

Heck, even then, it's fine for EX and Savage.

Indeed it is. My goal wasn't to say that MCH is not viable with Pugs, it's simply not as efficient as SAM.
If you imagine doing a race, you'll do a better time as a SAM but nothing stops you from finishing the race as a MCH.
The difference will be weak with a single run, but it can be a significant impact on multiple runs or on a messy run.

MCH remains viable and fine, simply not as efficient but nothing to justify giving up on MCH for SAM.