Is Maariv really optional?

The gemara in Brachos on 27b says that our mishna is teaching when it says that maariv "ain lu keva" it means that it is only a reshus. Tosafos on 26 says that this doesn't mean that it's optional, it just means that if another mitzva will pass then you should do that one and not this one. That really takes a lot out of the seemingly strong statement that maariv is only a reshus.

I was going to start looking around for other explanations but I got Rabbi Daniel Feldman's daily daf email and I was glad to see that he discusses it here.

Other rishonim note that ma’ariv is currently obligatory because it has been accepted that way in practice (see Tos. Yoma 87b, citing Behag, Rambam Tefilah 1:6, Bach O.C. 237).

Nonetheless, many understand that the reshut status does impact on halakhah. The Rashba (Resp. III, 288) explains the lack of Chazarat haShatz at ma’ariv as a consequence of this status (See also Rambam Hil. Tefilah 9:9). Rabbeinu Yonah (Berakhot 2b) goes further and suggests that if ma’ariv is a reshut, the amidah itself would be unnecessary; the representative version in “Barukh Hashem L’Olam” would be sufficient. The Rambam (Tefilah 3:7) writes that the looser adherence to the set times for prayer (i.e, the practice of early ma’ariv) is a result of this status.

He has lots of other interesting stuff on this topic.