We still have chance to reach 3750 progress without use of diamonds.
And how did you come up to that conclusion?
Rough calculations.
We can get about 10600 energy from the event for free.
In addition, we may receive the mail (~500 energy?) and collect incidents (~500 energy?)
If we played 4 runs every day, 140 energy per run, that will make 11700 energy consumption in total,
If you are lucky enough, you can make 70 runs in total.
140 x 70 + 20 x 21 + 40 x 21 + 80 x 7 = 11620
In first 10 spawns, breaking 2 blocks per spawn is not that tough.
With 14 spawns, 20 blocks can be taken. -> 20 progress per run, 1400 in total
By then, you can make 6 or 7 L4 keys. -> 20 keys per run, 1400 in total
And there is the daily quest. 9 x 21 = 189 keys here. 1589 in total
if you are able to convert this in gold chest ratio (>1.2), you get 1900 progress from here
Finally, progress from the first converted keys every day is doubled.
If you use this for the gold chest (23 per chest) every day, you get 483 extra progress at max.
1400 + 1900 + 483 = 3783
Some things upwards
* I feel it not difficult to clear 20 blocks by 11 or 12 spawns rather than 14.
* Maybe we get another set of cheap resets for day 22.
Some things downwards
* distribution of the keys and unmatch between the offers would not let us use up all the keys that we've got.
* It may be difficult to use 23 chest for the bonus every single day.
You have to be lucky enough to...
* receive the e-mail.
* get good incidents.
Conclusion
* You definitely
have a chance to get the golden upgrade without diamonds.
* You don't have to be
extraordinary lucky to get there.
* But you always have to use the correct strategies, contribute at max,
* and even then, you may fail due to lack of luck.
So, if INNO or Juber answers the question, they should say it's possible.
but I DO NOT THINK THIS IS THE RIGHT BALANCE OF THE GAME.