When you say "Je descends l'escalier", it means "I am going down the stairs" or "I'm going downstairs". It only refers to a flight of stairs, i.e., a single staircase. I suspect that different speakers may be confused by the fact that "stairs" can refer to a single staircase/stairway.
You are right, of couse --- but it is quite common in English to hear of climbing down a ladder, or a tree, or a mountain ---usually where there is effort requiring a descent facing the slope and with both hands clutching at any available support.! Trying to think of a word in English which means precisely that,.... maybe clamber?
Even though "I go down the stairs" is technically correct. It sounds like very poor "cave man like" English. If you hover your pointer over "descends," one of the options is "(I) am going down."
The preferred translation of this sentence, in my opinion, would be "I am going down the stairs."
It's not actually arbitrary - I'll see if I can explain. Avoir means "to have" in the sense of possession. We don't say "I am possessing two plates of rice," we can only say "I possess two plates of rice." But when you say "I am having two plates of rice" you mean "I am eating" or "I am ordering" two plates of rice. And for that meaning, in French you would use prendre: Je prends (I am taking).
For most other verbs, the two English forms can be translated interchangeably - ie Je joue: I play/I am playing
But it doesn't work for avoir in the sense of possession. That's why your answer wasn't accepted.
Hope that makes sense.
Good job explaining by nzchicago. Here is more on stative verbs when you are ready for more: https://www.thoughtco.com/differences-between-action-and-stative-verbs-1211141
The "-s" ending is a pretty common conjugation with "je" in the present for almost every verb whose infinitive ends in "-re," such as prendre "to take" (je prends "I take/I'm taking") and entendre "to hear" (j'entends "I hear/I'm hearing"). It doesn't say anything about whether you do it repeatedly or you're doing it at this very moment; context takes care of that.
It isn't wrong. It depends on your action in relation to the person you're addressing. If you're leaving someone upstairs it would be more natural to say "I'm going downstairs" - but if you're approaching someone who is on a lower floor "I'm coming downstairs" would be perfectly correct.
Not really. The ending of dessine rhymes with "scene." For descends, the "nds" are all silent, and the preceding vowel is nasalized. Also, I think the first "e" is slightly different because in dessine it is followed by a double s; but the main difference is the ending of the two words. Not similar at all.