I side with the English language speaking for itself and the affliction being instantaneous because there's no indication otherwise, the 'official' walk-back is just another example WotC changing clearly written works for the sake of balance without writing an errata, and the fact that the chatter about incubation times have no bearing on a fantastical world filled with magic - reality needn't apply as few 'reality' arguments makes sense in 5e with that context. (E.g., a restrained and prone target has the same difficulty shooting a bow at it's max range increment as a simply restrained or simply prone target within their inner range - theres no granularity = nonsense.)
Here's a fun thought. Since a lot of the interpretations within thise thread hinge on the English interpretation of clearly defined words whose context isn't really questionable as RAW (obviously the Sage has made their opposing RAI opinion known) let us look further into some English for the proposed solutions/modification of the spell debate. Start with the spell name and work up.
Contagion: From Middle English (late 14th century), from Old French, from Latin contagio (“a touching, contact, contagion”) related to contingo (“touch closely”)
This is in-line with the spells touch attack. However, further examination of the current English definition of the spell name (it's understood that the spell text is meant as the official D&D5e definition of the spell "Contagion" and that the English definition of the word isn't RAW relevant, but humour me a hot second).
First definitions of the word "Contagion" from multiple sources:
1. A disease spread by contact. (Wiktionary)
2. A disease spread by the close contact of one person to another. (Google)
3. The transmission of a disease by direct or indirect contact. (Merriam-Webster)
4. The situation in which a disease is spread by touching someone or something. (Cambridge English Dictionary)
5. The communication of disease from one person or organism to another by close contact. (Oxford English Dictionary)
The gist is the same: a malady, specifically a disease, that is spread by contact. Viola! That's precisely what Contagion the D&D spell does. The thing nothing mentions and it's implied in all definitions is that something contaminated with the disease agent passes said disease to an otherwise unafflicted individual through contact. As such, I say, keep the spell written as it is in the PHB (instantaneous onset with 3 fails to allow the disease to run full duration) with the following caveat: the caster is also afflicted (instantaneously) with the same disease. This seems to be in keeping with the spirit of the spell and its English gymnastics. Since the caster is directly touching the target they must be the conduit for the disease, therefore diseased, from whom passes it to the unafflicted target. Again, I'm aware no where within the spell text does the bolded text occur, I'm not saying this is an interpretation, but something to add to the modification of the spell debate.
In short, you want to disease a target instantly i.e. slimey doom? Great, no problem. You take on the contagion of slimey doom then touch the target; upon success, you infect and immediately afflict the target with slimey doom. On a failed touch, you're still afflicted. I'd even let the caster make multiple attempts to touch the (the same target upon failure) target so long as they remained afflicted themselves (without concentration, they've already paid a resource by taking on the disease no need to complicate things). After they've passed the contagion, should they wish to cleanse themselves before they succumb to it's later Con saves and/or potential for full duration... I'd say that's prudent and they should do so without consequence presupposing they have the means to cleanse themselves.
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