While they can still impeach without "proving" it, they shouldn't. But Trump should be impeached for either, or both, of the following:
1) Trump used his office to pressure a foreign power for his own political gain, as proven through the evidence provided. The transcript of the phone call, combined with witness testimony to date (closed door, and yesterday's formal start to the hearings), is ...compelling ...evidence.
2) Trump obstructed justice by preventing witnesses from testifying.
Any argument that the pressure didn't come from Trump (but: the transcript!) only means that he is incredibly incompetent (at leading our nation) rather than corrupt. Spoiler: It's both.