The Q hypothesis has issues. For such an important document or source, nothing is known about it. Its existence is not mentioned or hinted at in external sources. No trace of it has been found.
In addition, the synoptic puzzle can be laid in a self-consistent and "path-of-least-resistance way" by looking at e.g. author motives: Matthew writing for the Jewish community in Jerusalem; Mark describing Peter's preaching in Rome; Luke writing as a Greek doctor for a gentile audience (and John writing much later, clarifying and responding to the first heresies that had popped up).
So the Q hypothesis, aside from being a theoretical construction based on internal evidence, is not necessary either.
See e.g. "Case Against Q: Studies in Markan Priority and the Synoptic Problem" by Goodacre.
In addition, the synoptic puzzle can be laid in a self-consistent and "path-of-least-resistance way" by looking at e.g. author motives: Matthew writing for the Jewish community in Jerusalem; Mark describing Peter's preaching in Rome; Luke writing as a Greek doctor for a gentile audience (and John writing much later, clarifying and responding to the first heresies that had popped up).
So the Q hypothesis, aside from being a theoretical construction based on internal evidence, is not necessary either.
See e.g. "Case Against Q: Studies in Markan Priority and the Synoptic Problem" by Goodacre.