Buying legendary odds sounds really silly. Generally speaking, at level 8:
1.) You're all-in rolling to get strong ASAP because you don't have enough health to reach round 30 otherwise.
2.) You're economy rolling at 50 specifically to complete a level 1/2/3 three star hero or 2* important units in your team composition.
3.) You're rushing to level 9 because you have a good lead and/or your composition forces it.
I don't see any time where a player can afford to spend five gold to increase the chance that you'll see a legendary rarity hero. Seems like a complete waste of gold.
Somebody on Reddit had a comment I think has a lot of merit and I expanded upon a bit: Remove all legendaries from the pool. At level 8+ they should let you outright buy a legendary hero from the shop (call it the Secret Shop or something, lol) and it would have the stats of a 2* legendary hero. Maybe do a scaling gold cost depending on level, where the cost at level 8 is 50, cost at level 9 is 40, cost at level 10 is 30, or something. So now you have another option to all-in at 8 or 9 and finish your composition without the legendary rate RNG involved.
On another note, I grinded a bit this weekend and finally made Lord. BlessRNG, I would've had it earlier this week if not for a string of poor luck.