As far as selling crystals... because crystals are a set permanent purchase, making ships more easily attainable may boost sales short term but once everything has been purchased, the revenue will start to decline.
Making guild renown potions more "worth" purchasing while making renown gain continue to be relevant after everything has been achieved makes sense for long term sales. The new system makes guild renown potions much more worth purchasing for larger guilds who before have been avoiding them because they could not advance despite the use of renown potions. Smaller guilds that are not able to advance due to the limited membership and no desire to add in numbers, have less reason currently to purchase renown potions as it will not result in sustainable growth.
I am ok with tying decay with ship size. Bigger ship = increased decay, smaller ship = reduction in decay, smallest ship = 0 decay. At least this way players that want the highest rank "could" reach it and have some control over how much "work" they have to do.
If it's a system of no decay until a guild reaches a certain threshold, it makes sense to require battling against decay in some form in order to maintain the very best buffs and/or the biggest ship because realistically some guilds care only about the "best" buffs and once that is achieved will no longer purchase the gold amenities with turbine points or the renown potions for level progression.
I like the definition of smaller guilds being expanded however, if decay is reduced or eliminated entirely there is no longer a need to allot a bonus based on guild sizes.