Directionality is a mostly irrelevant consideration.
The fact that an antenna is 9db or 30db higher in one direction quickly becomes irrelevant with the vast distances of space. Antennas don't work like flashlights. They are more like a light bulb with a two-way mirror on one side that reflects 50% of the light and lets 50% of it through out the back. At VHF and above, things like mountains act like mirrors that reflect signals straight up (among other directions), as well.
You are somewhat wrong about AM... at least broadcast band AM is mostly only directional in the sense that there's dead zones straight off the ends of the dipole. They are shooting quite a bit of signal upward. Our ionosphere does strongly reflect and attenuate what would make it out to space in those bands though.
This goes toward your comment about the 50s and 60s... we have far more powerful transmitters in operation now (some VHF TV the better part of 1 megawatt!), and in bands that aren't reflected by the ionosphere. If anything we are getting louder and louder.
Unfortunately the first thing they might see of humanity is free-to-air broadcast TV, and just assume that we are all complete idiots.