Yes if the number of HotM do not increase, but every month, a HotM enter that pool, in 4 years time, that is another 48 HotM, which means the odds of specific hero is halved.
We could calculate how long until the expected frequency reach 1 by accumulating the odds of each training done.
Currently there are 49 heroes but by the time we can use it (around September at the earliest without gem skip), it will already become 52.
At September the expected frequency of a specific heroes per training is 0.05/52 = 0.000962
At October it will be 0.05/53 = 0.000943
Each month we have (365/12)/7 = 4.345 training chance.
Accumulated expected frequency from September = 4.345 × 0.000962 = 0.00418
From October = 4.345 × 0.000943 = 0.0041
Total expected frequency from those 2 months = 0.00418 + 0.0041 = 0.00828
How long until the accumulated expected frequency reach 1?
n months where n is:
1 = 4.345 × (0.05/(51+1) + 0.05/(51+2) + … + 0.05/(51+n)
1/(51+1) + 1/(51+2) + … + 1/(51+n) = 4.60274
(How to write mathematical symbol here…)
I will just use excel to calculate the result…
And the result is n = 5087 months… or around 424 years.
In another words, on average, you need 424 years to get Gravemaker from HA. You can be extremely lucky by pulling it on your first retraining, but majority of retrainers won’t get him from HA even until the end of times even if they keep on retraining.