This type of crystal lattice is not meant to be the only source of selectivity in a circuit. At very high frequencies, the parasitic capacitances of the crystal holders and electrodes simply pass everything.
It would be more typical for this sort of lattice to be incorporated into an IF chain that also has ordinary LC circuits to provide the required attenuation farther away from the desired passband.
The equivalent circuit for a quartz crystal is something like this:
simulate this circuit – Schematic created using CircuitLab
The components across the bottom represent the mechanical resonance of the crystal itself, while the capacitor at the top represents the capacitance of the electrodes and holder. Typical values are:
- C_ser: 10s of fF (yes, femtofarads, 10-15F)
- L: 10s of mH
- R: 10s of ohms
- C_par: 10s of pF
The crystal has a series-resonant frequency based on just C_ser and L. It has a relatively low impedance (basically just R) at this frequency.
It also has a parallel-resonant frequency when you consider the entire loop, including C_par. Since C_ser and C_par are essentially in series, together they have a slightly lower capacitance than C_ser alone, so the parallel-resonant frequency is slightly higher. The crystal's impedance is very high at this frequency.
But at frequencies much higher than either of the resonant frequencies, you can see that the impedance of C_par alone will dominate, and this just keeps decreasing with increasing frequency.