Are you sure? Its melee weapons is 3 attacks at 2" range, with 3/3/-1/d3 which isn't too bad IMO.
In fact it is better than the damage output of some support heroes or elite Infantry:
- Saurus Oldblood carrying a Suntooth Maul
- Astrolith Bearer
- Lord Castellant
- Orruk Brute armed with a Gore-Choppa (the special weapon)
Granted, those are not too great in melee compared to real dedicated melee heroes, but still better than most wizards at least. The fact that it has rend helps a lot.
So I wouldn't exactly call the Basti's melee capabilities weak, especially since he will most likely be there for more than a round. Most melee heroes that have better attacks have less wounds and/or a worse save (a Savage Big Boss for example), OR they are almost as expensive as he is (a Lord Celestant on Dracoth for example).
Those are all significantly cheaper than the bastiladon. If you spend the same amount of points a bastiladon costs you get a far better melee force.
Anyways, I'm not saying the bastiladon melee is necesary weak. I'm saying that so much of his firepower is tied up in his ranged weapons that he needs it in order to actually go and block some major threat (which is his main puprpose), say another behemoth or a 40 man horde of buffed up guys, and come out on top. Without his ranged weapons he simply losses so much firepower that apart from just standing in the way he won't do much.
To put this into perspective a bastiladon with solar engine vs 40 skaven clanrats in melee:
Currently he could do the following:
Your turn:
Shooting:
Solar :2D6/4+/3+/-1/2 averaging: 4.66 damage
Javalins: 4/5+/4+/-/1: averaging: .22 damage (after saves)
Melee:
Tail: 3/3+/3+/-1/D3 averaging: 2.66 damage
Total: ~7.5 damage
Battleshock: an average another 7 flee
Opponents tun:
Melee:
Tail: 3/3+/3+/-1/D3 averaging: 2.66 damage
Battleshock: an average 1 flees.
Total losses per turn: about 18. So it takes about 2-3 turns to wipe out the entire unit of clanrats on average depending on where you fall on the bellcurve.
If we aren't allowed to shoot in melee we are only left with our melee attack resulting in 3.66 losses on average in both our opponents and our turn. Resulting in roughly 7 losses per turn meaning it now takes roughly 6 turns to wipe out a unit of clanrats, and even then only barely (1 unlucky roll and it takes an entire extra turn...). hell given how much time he now needs to actually kill the clanrats they might even be capable of overrunning him. With a 3+ save he should fall after about 24 succesfull attacks. That'd take about 144 skaven spear attacks (not counting the horde bonusses). with them Dying/fleeing at 7/turn this would be viable as long as at least 10 skaven reach him with their attacks each turn. Which should be more than doable.
Hence the loss of his ranged weaponry would mean a bastiladon would struggle to actuall win a fight against even a mere unit of clanrats on his own. That is how much of his firepower is tied up into the solar engine. Hence completly removing his ability to shoot in melee seems like a very harsh nerf to me, to the point where he wouldn't be all that good anymore at his intended purpose.
For clarity, a similar case can be made for the ark of sotek, but seeing as that's an AoE attack and not single target the comparison is a tad more complicated to do fairly.